It depends on where you plan on playing the most. 50mm is more portable and lets you easily play on smaller surfaces in casual settings like a living room or table at a cafe.
If you're looking for more serious play I would not go below 53mm.
What square for this set?

Hi,
This is 50mm. Should I go for 55mm? The bishop's base ideally fits between the king and queen, but it's hard to sweep it across without slightly touching the pair (unless, of course, I lift it a bit).