Here are all the sizes available: Atlas Code 83 Chart. I think there is a chart like this for each library on the RR-Track website under Track Libraries.
There is also a 'Custom Curve' selection which I think allows you to make any size you want, but I have never tried it? I only have the Atlas Code 100 HO library, which has about the same selections. Got it for my grandson, I really haven't used the HO library much, other than to just look it over.
There is a 'Generic' HO library that has wider radius curves, I think up to 30" and it also has the 'Custom Curve' selection.