This is a matter of taste, and there are many good solutions.

I'd however recommend using a comma:

• --size=45 = exactly 45
• --size=45, = at least 45
• --size=,45 = at most 45
• --size=20,45 = between 20 and 4t

However, using two options like --size-min and --size-max might be better in some cases.

Update: I'm not very familiar with Windows shells, so this might not be a good solution on Windows.