Does anybody know how big of a tire will fit in a voodoo wazoo cross frame?

Which "generation" is the frame? Up through the '99 model year, you could run just about anything; I have WTB Allterrainasaureses on one of mine (a nominal 40c tire, but the huge edging knobs make it more like 44c). When VooDoo went to the more "Euro" frame geometry in 2000, the seat stays got a little narrower--I just did a quick (admittedly imprecise) comparison and there's about 1/4" less room on the 2000. Still, you should have no trouble getting a 38c, maybe a 40c in there, regardless of the year.