limiting factor is not sparging, it's mashing
at 1.25 qts/lb, you could fit 32 lbs of grain in a 52-quart mash tun, according to rackers
shape of the tun and batch size are irrelevant to how much the tun can hold, but you should measure to make sure the tun actually holds 52 quarts
my 70 quart tun measures 24.5 wide by 12.5 high by 10.5 deep = 3215.625 cubic inches = 55.68 quarts