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