sheag123 :
in some m-atx cases some atx psu's fit and some dont. its best to find the case you want and make sure the motherboard, cpu cooler and psu will fit in that order or find the cpu cooler and mother board you want and find a case that fits those components. it depends on your setup. i know i have given you too much information but just trying to save the back and forth. its best to give as much info as you can. and most times a quick google will answer 90% of the questions you might have but np
i had it easy one time, i had 2 towers about the same size(i suppose same form factor), almost the same or same not sure, one of them had bad psu so i switch psu but it did not fit in the tower, still i plug in the psu to motherboard and it worked, so i ordered the psu based on the computer model
now how do i make sure it fits in the mid tower based on what? you said form factor is not the answer