Brassard suggesting measuring by looking at the lube on the dilator, and basically using it like a dipstick.

In the beginning, I could see in the mirror that my labia were so swollen that some lube was rubbing off on them and getting left on the dilator; however, now, if I spread them apart a little with my hand the lube is a fairly accurate measure because I put it on the tip and it gets dragged along to the edge of the vaginal opening as I insert it. If I do try measuring by looking at the dots lining up with the end of the vaginal canal, I get roughly the same depth, too, so that's basically double-checking each method against the other.
Oddly enough, I was 5.5" then and I'm about 5.5" now, but since part of that was swelling I presume I've actually gained a little bit of depth. It seems perfectly adequate, particularly since the average penis is about 5" and not all of *that* will be inserted if it's attached to a human with bones and joints and whatnot instead of a plastic dilator. Width seems more relevant when it comes to penis size (ask my wife, who complains that I was thicker than the biggest dilator and it hurt her sometimes...), anyway.