Example sizes that will work
180x180 (3 x 60 = 180)
240x240 (4 x 60 = 240)
900x900 (900 / 60 = 15) no remainder
420x600 (okay as long as it's divisable by 60)
Example sizes that will NOT work
192x192 (3 x 64 = 192) must be 60, not 64
520x520 (520 / 60 = 8.6) must divide evenly