The formula for calculating the file size is this:

MDL_file_size = vertices*12*frames + faces*6 + skin_vertices*8 + skin_faces*6 + skin_width*skin_height*bit_depth/8;

That's the formula used for calculating the file size of any model, no matter how many frames it has. This assumes vertex animation. For bones animation, it has very few differences:

MDL_file_size = vertices*12 + faces*6 + bones*2*frames + skin_vertices*8 + skin_faces*6 + skin_width*skin_height*bit_depth/8;