Chris,
A standard small curve in HO is 18" radius; adding in extra for the base of the EZ-Track, you need roughly 38" for the circle.
You can go two alternate routes if you are willing to give up the EZ-Track roadbed.
1st, you can find some 15" radius sectional track: t he diameter will be close to a circle of O31.
2nd, you can get flextrack and make the circle the diameter you need.
An important consideration, just as in O, is minimum radius for the equipment. MOST B-mann locos will handle 18" radius. MANY will do the 15". SOME will do 12". A FEW will do as small as 10" or maybe a wee bit smaller. Their freight rolling stock will easily do the 15"; some will do the 12". You can custom build cars to do the 10". The passenger equipment will go a little smaller than 18".
There is a detailed chart out there on the web listing minimum radius for each B-mann piece: http://www.npcrr.net/On30/On30..._Car_Radius_pics.pdf
The charts lists B-mann's numbers and actual test data from users.
