Hi Jim,
If you're using a 54mm figure, it will be (approx) 1/32 scale. This means that the tank will be a 1/35 scale kit. The figure will be slightly overscale, but unless you're going to have other figures in close proximity, the difference will be negligible.
Of the Stuart kits available, Tamiya's old M3 kit is usable, though it has some dimensional issues because it was designed to be motorised, so it had to be able to fit a motor, gearbox and batteries within its hull pan. It also has tracks with the end connectors moulded incorrectly to each track block, so they don't actually connect the track links to each other. It does come very inexxpensively though and is a quick and easy build.
Though it also has some issues, Academy's much newer kit has better detail and includes an interior. .