I am trying to buy a used a rheem heat pump but not sure about its size. The serial is:
RQMA-A303JK000.Is it a 2.5 ton?

No such model number listed exactly as you supplied. The number series does contain 30 in it which indicates it could be a 30,000 BTU, 2 1/2 ton unit but without a working model number cannot tell for sure.

You are correct, I just took a look at the unit and the compressor was a copeland ZR30k-PFV. It was a good unit but wron size for me, I need a 3 ton. Thanks for your answer

Better to find out before you buy.