restrict width and height on mop initialization

Hello,
I would like to create a mop with 0 input and 1 output and restrict the matrix info to be "4 char 640 480".

How can I restrict the matrix width and height?

in order to set the type and planecount I use jit_mop_single_type and jit_mop_single_planecount. it seems like there is no such utility function for the dimensions.

I tried:

p = jit_object_method(mop, _jit_sym_getoutput, 1);
m = jit_object_method(p, _jit_sym_matrix);
jit_object_method(m, _jit_sym_getinfo, &info);
jit_attr_setlong_array(&info, _jit_sym_dim, 2, dims);

but m returns null.

Any suggestions?
Thanks a lot

PS: I realize too late that this post belongs to the dev section. I apologize for the mistake.