This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 13}\otimes\mathrm{Sym}^{ 16 }$. The following table gives the dimensions of various subspaces of $M_{13, 16}(\mathrm{Sp}(4, \mathbb{Z}))$.

Total8
Non cusp0
Cusp8