$\mathcal{MA}$ in terms of $\mathcal{PCP}$
If you want to restate the definition of MA in terms of PCP, you need another parameter for PCP, namely the proof length.  MA is clearly the same as PCP with polynomial randomness, polynomial queries, and polynomial-length proofs.  Usually the proof length in PCP is not restricted (that is, it is bounded only implicitly by randomness and queries), but this is insufficient to restate the definition of MA.

If you are looking for some characterization of the form MA = PCP(q(n), r(n)), which is not just the restatement of the definition of MA, then I do not think that any such characterization is known.