[verb (plain)] + のは + [noun] + くらいのものだ [i-adj] + のは + [noun] + くらいのものだ [na-adj] + な + のは + [noun] + くらいのものだ [verb (dictionary form)] + くらいのものだ
くらいのものだ means "about the only one": X is just about the one case that fits, and you'd be hard put to find another. The usual frame is 〜~のは xX くらいのものだ, with the description in a のは clause and the noun before くらいのものだ. ぐらいのものだ is the same.
The verb or adjective in the のは clause is in the plain form; a な-adjective keeps な. It is written or careful speech, and the tone is usually amazement or exasperation at the one exception.
After a verb or a word like これ, くらいのものだ means "that's about all": what is left is small.