My granddaughter used an 80 for most of her pool work and a 63 all open-water work and subsequent dives (15+ to date). She could handle the 80 in the water, but not out. The 63 worked fine and she, too, is a squirt (75-80#). Biggest issue we had with her comfort and performance underwater was the BC - we tried a dozen S and XS BCs and all were too big. Fitted my Oxycheq 18# wing/soft pack for her, but ended up finding an Oceanic kid's BC on eBay and it worked great. Her Mom (also small) used the Oxycheq. Highly recommend spending the time to find a good fitting BC above all else. Next biggest issue was finding fins and boots that fit properly. Tough to spend a lot of $ on something she'll grow out of in a year, but worth it for her to have a good experience UW. I used to keep 50s on hand when I was teaching, but find the 63/65/67 to be a more useful all-around tank. By all means, go for a 40 if you have other uses for the tank, but understand the limitations imposed by such a small tank. Also understand that you'll not find 40s to rent anywhere you travel. Much better luck finding 63s.
---------- Post added January 14th, 2013 at 11:17 AM ----------
One more thing (well, actually, three) - miflex regulator hose, a short Octo hose (standard reg hose) and a small mouthpiece increased comfort considerably.