Stock is 26" diameter. However, Subaru seems to set them somewhat conservatively, so 26.5" to even 27" is better. I'll note you can fit any tire with a section width up to 9.5" in the wheel well on a +48mm offset rim, i.e. 235 and even some 245 wide tires. However, you're stuck with 225 and maybe some 235 tires with the 7" wide stock rims. 205 to 225 will be more common for that rim width.
Stock 03 WRX rims are 16 x 6.5, just like Forester wheels but with a different offset. I'd normally recommend 225/60-16 but I can't say for sure that they will not rub.. I'd have do do some research. Will a 16 x 6.5, 55 mm wheel with a 225/60-16 tire rub on a 99 Forester w/ WRX struts?