Which E-Z Track (nickel on grey bed) product would I purchase to make a 24" circle for N scale train. I have the Frontiersman & wish to put it on a smaller table than the 36" that came with it.
According to Bachmann, the "footprint" of the EZ Track included with the Frontiersman set is 34" X 24".
http://www.bachmanntrains.com/home-usa/products.php?act=viewProd&productId=24
If you want a 24" circle, simply remove the four 5" straight tracks.
If you're looking for EZ Track with a tighter radius than the 11.25r included in your set, you won't find it.
Bachmann appears to be content to offer only two different curved radiuses for the EZ Track system, those being 11.25r and 19r.
If it's your wish to have a tighter radius, than Bachmann currently offers, you will need to look to another manufacturer.
Your Bachmann N scale equipment (loco's and rolling stock) will work on any other manufacturers N gauge track.
Good Luck.