I just got (mostly) done with this very small (21.5" toptube) frame for Sheri, and thought I'd post up a picture for her. Special bonus: Snickers, looking confused, poses as well.
The frame also reminded me that I need to make some things clear about who will, and won't, fit well on 29ers. There are a couple of hard and fast measurements you can take to quickly determine how feasible a 29er is for you:
-Measure your cockpit length (huh, huh, he said...). This is easy - just take a measuring tape or long ruler and measure from the center of where your stem clamps to the handlebar back to the center of the seat rails, level with the ground. If your bars are higher or lower than your seat, measure to where the saddle clamp *would* be if they were pefectly level. If this number is less than about 24.5", you can probably forget about riding a 29er (though small feet or short femurs that necesitate a steep seat tube angle can help a bit). Think about a 650b or 26" bike.
-Measure your handlebar height, from the ground to the grips. If this number is <36" or so, you won't fit on a suspension corrected 29" bike easily. Not a problem, obviously, if you want to go rigid-specific - in that case, 34" is the magic number.
-Measure your inseam. Standover *can* be a problem for smaller folks, though the first two problems already mentioned are usually bigger hurdles. Honestly, people are a little too hung up on standover - you will never, ever crash such that you land on the toptube, and if you do, a lower toptube just means you'll have farther to fall onto it. If you can mount and dismount the bike comfortably, you're good to go. That said, if you've got a 28" or shorter inseam, a 29er may not be a great idea for you unless it's non-suspension corrected.