You can get amp kits that include EVERYTHING. What is the model of the amp. Judging the amps real power needs from its peak power numbers is not that realible. But yea, 15-20ft of 4 gauge for 1000 watts of continous power is about right.

Dont forget to include the length of the power wire when picking the gauge size. And speaker wires are used to connect the sub to the amp.