Based on Jim Muller's response you should be good with 700c. The question then is how wide a tire you can mount on those rims. To do that you need to know the inside width of the rim. You can check that the rims are the same outside width with calipers (they usually are). To get the inside width you need to deflate one tire. Then you can go to one of several sites with charts of the range of tire width appropriate for a given rim width. And you can likely get away with exceeding the upper limit by a few mm, but probably not a whole lot more. Perusing the tire manufacturer's web sites (Continental, Schwalbe are the two most popular around here, but there are others) will give you an idea of what's available in that size and relatively knobby tires.