Can I estimate what CPLD I need?
Can I use the CPLD I have and then, once I have tested that the design works properly, tell ISE to synthesize it for smaller CPLD models to know which one could support the design?


Yes. This is a standard design practice.

One caveat to be aware of is that a more tightly "packed" design may have different timing characteristics. If your design is operating close to timing limits on a larger CPLD, it may fail timing on a smaller part, even if it fits.