45Ah is how much capacity the battery has - in theory, it can supply 45 Amps for 1 Hour, or 1 Amp for 45 Hours (or 2 Amps for 22.5 Hours, or 3 Amps for 15 Hours, etc). There’s absolutely no problem (apart, perhaps, from a tiny weight penalty) in getting a higher Ah battery, as long as it will physically fit.
CCA - Cold Cranking Amps - is how much power can be provided short term to start the car. Simplistically, the bigger the engine the more it will require. So a 5 litre BMW M5 will require a higher CCA than a tiddly 1.8 MX-5.