This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 15}\otimes\mathrm{Sym}^{ 30 }$. The following table gives the dimensions of various subspaces of $M_{15, 30}(\mathrm{Sp}(4, \mathbb{Z}))$.

Total52
Non cusp0
Cusp52