I think a 10 amp will do fine.
40 Ah is not equal to 40 A. I think you confused that.
Most of cars have batteries around 40 Ah, not 40A. 40 Ah means that the battery can source 40 A for 1 hour, it's a "capacity" measurement.
A 40 A tester will be bulky, and will not test well. Because you can have a 40 Ah battery with 800A cold start current, so 40A will not be sufficient to test that.
There are several ways to test the battery charge.
The voltage, the internal resistence (this one you do with load), charge retainment...