Usage patterns
〈物〉のサイズが合うfor the size of something to fit
These shoes don't fit my size.
サイズを測るto measure the size (of something)
I had the store clerk measure the size of my feet.
サイズを直すto alter or adjust the size (of clothing, etc.)
I had the trousers altered to fit.
〈物〉のサイズを選ぶto choose the size (of something)
It's hard to choose the right clothing size online.