Watts is not the ideal value we need. If that is all you have, we need volts too (I'm assuming 240V). Is there anything that says Mimimum Circuit Ampacity (or MCA), Rated Load Amps (RLA), Full Load Amps (FLA), and/or Maximum Fuse/Breaker? The value we need is MCA, but if that isn't there then we need the motor FLA and the compressor RLA.
From the few AC compressors I've looked at recently, a 2.5 Ton 240V unit would have an MCA of 18 to 23 amps which requires a #10 copper branch circuit (#12 is OK if MCA is less than 20). The breaker installed on this wire would probably be a 25 or 30A depending on what the manufacturer says. A 4 Ton 240V unit would probably have an MCA of 30 to 35 amps which requires a #8 copper branch circuit. The breaker for this would need to be a 40 or 50A breaker again depending on what the manufacturer allows.
You can upsize the wire to the customary sizes for the breaker if you wish -- will be a more efficient installation that way but cost a little more.