Hey, look at it like this,
Statisticly speaking, the average depth of a womans vaginal canal is about 4", abit more during intercourse due to sexual arousal but still it's not that deep considering the average length. And as mentioned by nerdygirl, continuously smacking into the wall won't necessarily be comfortable for either one... unless you're into that I guess... So one can assume that it all comes down to girth rather than length since average length would be sufficient!
That said however... I still find myself wishing sometimes that I were part elephant...