The use of testosterone replacement therapy (products such as Androgel) sometimes causes testicular atrophy, that is, shrinking of the testicles. Therefore, I am surprised that a doctor would suggest that using testosterone would increase testicular size, since the documented effect is quite to the contrary. The ability of steroids such as testosterone to shrink the testis is well-known among men who use heavy doses to enhance physical training, and there are lots of jokes about little balls on steroid users. Testosterone use may cause enlargement of other things, such as the prostate, or quite interestingly, the breasts.
I use Androgel because I have marginally low testosterone. At one point, while experimenting to find the optimum dosage, we had the dosage too high, and I observed a noticable shrinkage of my testis, although these were quite amply sized to begin with. When we halved the dose, I returned to my normal size. A friend of mine is on very heavy doses of testosterone as part of his HIV treatment. He's noticed that his testis, which were small to begin with, are now something like grapes. (Although he is self-conscious about this, no one notices, because of the lovely, thick 8.5" uncut meat above them!

However, at "normal" doses, Testosterone seems to have little effect on testicular size, while heavy doses seem to cause shrinkage, not enlargement. So, I wouldn't think that a small dose would make any difference. I am now using what might be called a small dose, and as I siad my testis have regained and maintained their original size, although this was not small to begin with.
To learn more about the effects of Testosterone, read the patient and physician information sheets and colateral material provided by the drug companies selling the testosterone products. Information from the makers of Androgel appears at the links below:
http://www.solvaypharma.ca/en/products/hcp...el_HCP_Mono.pdf (see the top of page 12 for mention of testicular atrophy (shrinkage)).
http://www.androgel.com/pdfs/androgel_prescribing.pdf
http://www.androgel.com/healthcare/qanda.htm