One huge concern that some men have when going on HRT (testosterone therapy or TRT) is testicular shrinkage. Will you go from a mighty acorn to a shriveled raisin? The concern is valid, because adding in external testosterone shuts down the HPT axis significantly and the testes can reduce in size a little. Of course, the question is "how much?" and "is it worth worrying about?" Below I will present some evidence that the shrinkage is not as much as most men think. Let's look at a couple of studies that show this:
1. Healthy Men Given Testosterone Undecanoate. This is an interesting study that came out of China, where they looked at testosterone undecanoate as a contraceptive for men.  The shorter esters seemed less desireable for this purpose, so they looked at undecanoate, which some of you will know by the name of Nebido, Aveed or Reandron 1000. (This was fairly recently approved by the FDA in the U.S.)
The protocol for the testosterone undecanoate men was as follows: a 1000 IU initial dose and then 500 IU monthly for the next 11 months. The goal was to reach azoospermia within six months, something this protocol did indeed achieve. In case you doubt the power of testosterone therapy to shut down fertility, consider the fact that this protocol was 95% effective in reducing pregnancies, which is a pretty decent number compared to many contraceptive methods. I should also add that this study was done on healthy younger males (ages 20-45) using an undecanoate immersed in tea seed oil.
CAUTION: Other forms of testosterone therapy may not be as effective. This study stated that Testosterone replacement therapy results in decreased serum gonadotropins and intratesticular testosterone, and impairs spermatogenesis, leading to azoospermia in 40% of patients."  In other words, don't count on TRT as a form of contraceptive..
So what did this do to their total testosterone levels? Well, first of all, we should probably talk about the dosage used in the study. Nebido / Aveed / Reandron is generally a 1000 mg per shot and is given 6 weeks after the initial injection followed by an every 10 week protocol. (Of course, physicians adjust according to their patient's lab results and symptoms.) Therefore, the average is roughly 100 mg per week during the maintenance phase out in the "real world." However, in the study above, the maintenance dosage is more like 116 mg per week on average. So the dosage is a litle higher, but this is a realistic dosage and close to what is used mostly in clinical practice from what I have seen.
The study does not directly give the baseline average pre-undecanoate testosterone levels of the participants. However, they do state that total mean testosterone levels increased by 133% and a chart shows that the change was between about 550 ng/dl and 610 ng/dl. The point is that their testosterone was indeed significantly raised as expected. Therefore, one would expect that their testicular volume would greatly decrease, right? Actually, what happened, though, was the opposite: testicular changed minimally from 38.9 ml to 34.7 ml. This is only a 10.8% drop in testes volume and would probably not be visible to the typical female out there, unless she has been trained as a PA for a urologist's office!
To explain how small of a difference this is, let's pretend that the testes are spherical. Of course, they are not, but that approximation will be good enough to make a point. If you run the numbers, you'll see that the radius would change only from 2.10 to 2.02 cm, which is only a 9.6% difference. Thus, the actual side to side change is smaller still. Also, one cannot use the argument that this was not a long enough study as it lasted for one year. That is plenty of time for the testes to reduce in size.
As a side note, I asked my wife before going on TRT if any shrinkage would bother her. She looked at me like I was from another planet and said that she had never heard of any woman talk about the size of a man's testicles. Of course, I am sure that someone out there has a contradictory story, but I believe this issue is mostly a concern inside a man's head.
2. High Dose Testosterone Enanthate. Testosterone enanthate has also been studied as a contraceptive in healthy men. For those in the U.S., the enanthate ester is very similar to cypionate both in mg delivered of testosteorne and half life. Most men on cypionate on the Peak Testosterone Forum are injecting about 100 mg per week. In this study, they used 200 mg of enanthate per week, which is a very high dose. However, they were probably doing this to make sure that the men really shut down their sperm production. Due to the fact that they were using a rather high dose, testicular volume decreased by 24%, significantly more than in study #1. Again, this is due to the unusually high dosage.
Again., the point is that more reasonable doses of testosterone will not yield much loss in testicular volume. However, your HPT axis and fertility will be very significantly decreased. LH and FSH numbers both drop dramatically on TRT attesting to this fact.
SOLUTION: In spite of the fact that shrinkage does not directly matter, I realize that many men will be concerned over the issue and may want a solution. Low dose HCG can be added to testosterone therapy if desired to add volume to the testes. Common dosages are 250 IU two times per week or 500 IU two times per week. I did the twice-per-week 250 IU protocol, and it worked very well. Some men will have to go up to 500 IU though. It should be pointed out, though, that HCG can restore your fertility as well. In fact, a Dr. Lipshultz in Texas developed a protocol that will restore fertility using HCG in most hypogonadal men on TRT. See my on Testosterone and Fertility for more information. HCG will also boost libido in some men as well.
However, it probably should be pointed out that HCG will usually not bring the testicles to their maximum size. To do that usually requires some HMG, which stimulates FSH receptors in the testes. Note what this Medscape article says:
"Quite apart from its reproductive implications, the psychosocial impact of small testes can be considerable, particularly if the patient becomes sexually active. Testosterone therapy does not change testicular volume (TV) whereas hCG gradually augments TVs to a more satisfactory although suboptimal 6 8 cc volume. Further enlargement necessitates adjunctive FSH preparations." 
1) J Clin Endocrinol Metab, 2003 Feb, 88(2):562-8, "A Multicenter Contraceptive Efficacy Study of Injectable Testosterone Undecanoate in Healthy Chinese Men"
2) Medscape, "What is the Optimal Therapy for Young Males with Hypogonadotropic Hypogonadism?" T. S. Han; P. M. G. Bouloux Disclosures Clin Endocrinol. 2010;72(6):731-737.
3) J of Urol, Feb 2013, 189(2):647 650, "Concomitant Intramuscular Human Chorionic Gonadotropin Preserves Spermatogenesis in Men Undergoing Testosterone Replacement Therapy"