The holes are spaced for a brushless motor with 16 x 19mm spaced holes. The holes are 3mm and counter bored for the screw head to recess. They are designed for socket head cap screws. The screw head size is 5.5mm. After printing, you may or may not have to slightly drill with a 3mm drill bit. However, you may do fine simply screwing in the fastener and allowing the threads to ream out the hole. The base of the holes is 2mm thick giving the screws enough material to be securely tightened without the possibility of damaging the part. The 'skid plate' in the center is 10mm diameter x 3mm height. The outer diameter is 28mm with a main thickness of 5mm.
If you would like to have a custom size skid plate, leave a comment and let me know! I will be designing more in the future so check back!