2002TransAmKid09

i have 2 of the original 17" polished rims that are chipped and wanted to go bigger in the back, i was thinking 19 inch Ruff Racing R280, i have people telling me go 20 all around but i only want to replace the 2 bad ones and throw the good ones up front, what tire should i use and are those rims sufficient? im obviously new to this but i currently have yokohoma type S on all 4s so i hate to see 2 fish hooking tires go down and not get them in 19s or 20s.
WHAT THE HP LOSS IN LARGER RIMS? HOW WIDE CAN I GO?
IM RUNNING STOCK ENGINE, STOCK (10 BOLT) REAR AND ALL.
WHAT RIMS WOULD YOU ADVISE, and please dont say torque thrusts.