I do not consider this concept intriguing as much as I consider it misinformed.

While it may be true that a longer penis would usually place the tip of the penis deeper inside the vagina during climax, I concur with TheLordScribe's observation that semen is not seeped, but rather ejected (compare throwing a dart at a dartboard from 10 feet as compared to 5 feet; although, you're not really aiming at a target here. Well, you are, but if you miss it, you've got bigger problems than penis size.)

There are a number of other factors that have far greater influence on whether one of your sperm makes to to Checkpoint Charlie, a few being the acidity of the vagina, how long its been since you last ejaculated (multiple ejaculations in a short period produce less and less ejaculate with each orgasm), and your overall health (if you have a fever, you produce fewer sperm.)