A 30 breeder would work just fine for one or two pairs of frogs. From what I am told, getting them to breed is not hard (they will do it on their own if kept healthy and happy) but raising the tads is quite difficult.
Of course, if you want to hasten things, you could expose the female to the urine of a pregnant woman; this will induce her to ovulate. Fifty years ago Xenopus were a common pregnancy test.