Not trying to be a wise guy here, but if your considering these two skimmers, I am assuming you did some research on them. The footprint size is usually on the info page you are reading about the skimmer"s specs.
OPE 250= 16 x 16 footprint, 24 3/4" high
OPE 300= 13 x 17 footprint, 24 3/4" high