(A) Early two-leg tadpole, (B) two-leg tadpole, (C) four-leg tadpole, (D) four-leg metamorph with tail.

If you inhibit thyroid hormone function, metamorphosis will not occur (see Brown et al., 1996 and references therein)

In most frogs, the larvae (tadpoles) eat, but are sexually immature and so cannot breed.

I assume that as long as you block the thyroid hormone pathway, you will block metamorphosis, but I cannot find any published studies on how large a tadpole will grow under these conditions (or how long they will survive).

I suspect that it will not grow anywhere near as large as an adult (mature, that is, sexually active) frog, although late stage tadpoles can be as large as froglets are immediately after metamorphosis.

Froglets do grow to be much larger, however.

image of Xenopus laevis going through metamorphosis, from Freeman & Rayburn (2004)