I'm just wondering what the dimensions of an mgh file generated by
mris_preproc with e.g. 300 subjects should be?

Should it be 163842x1 or 163842x300 ? I would assume that it should contain
a vector for each vertex, with the same length as number of subjects that were listed in the fsgd file used with mris_preproc. But when I run mris_preproc on 300 subjects I get a 163842x1 file (as read in Matlab).