The answer is yes and no.
Let's start with some obvious observation. Unlike a man's scull or brain, his penis is an organ with no fixed size. Sometimes it's bigger, and sometimes it's smaller.
The size of the penis at any given time depends on a multitude of factors: genetics, body chemistry, erotic thoughts, diet, you name it.
Or the adrenergic system. Yohimbe is an alpha-2 adrenergic receptor blocker. Or rather, yohimbine is. Yohimbe bark that does not contain the alkaloid yohimbine is a nuisance. It will cause a headache rather than an erection.
By being an alpha-2 adrenergic receptor blocker, yohimbine, (yes!) does increase penis size. But only for as long as yohimbine is circulating in a man's body. When the yohimbine is cleared, the penis will retract to its flaccid baseline size, or, for a few hours, even be smaller than baseline.
I assume this is not what you had in mind when you asked whether yohimbe increases penis size. You probably wanted to know whether there is permanent growth.
For permanent growth, you will need another strategy.
You can use Tongkat Ali extract. Tongkat Ali alkaloids stimulate the Leydig cells of the testes to produce more testosterone. This spurts penis and testicular growth, which at least is semi-permanent. Size gains will persist for some time after the regular ingestion of Tongkat Ali has been stopped.
Leydig cell activity is essential for penis size. Which is why bodybuilders who use anabolic steroids are fools. By supplying exogenous androgens, they totally shut down Leydig cell activity, thus causing substantial testicular and penile shrinkage. Their upper bodies may look like those of heroes, but below the belt, they appear as prepubescent boys. In bed, they are a joke.
For any man who is concerned with manhood, the increased testosterone desired for muscle gain has to come from his own Leydig cells (through stimulation by Tongkat Ali or synthetic agents that exert a similar effect). Additional benefits are increased pleasure and functionality in bed.
The only approach that works for permanent gains of baseline as well as erect penis size is a penile exercise program.
A good number of such programs are sold on the Internet. Access costs between 30 and 100 US dollars. The better programs include videos on all exercises. I consider videos essential, as the various exercises are difficult to comprehend from written descriptions.
The cheapest quality exercise program I know is offered at MensPride.net. They charge 30 US dollars, have videos on all exercises, and also answer email (not the case with most other programs). MensPride.net are cheaper than others because they do not market their program through an affiliate program. For most other programs, the fact that they pay around 50 percent commissions for references by so-called affiliates jacks up the price. Of course, this commission, at the end of the day, has to be paid by the buyer. (ve*n)