hey mate, 8" will be wide enough, but u can go up to 9" if you think you might want to go wider in the future, 9" will hold a 255 easily, or u simply prefer a more direct feel (not such an issue with semi slicks due to their stiff sidewalls anyway).
as far as clearance issues, the mac strut on the S13's is known for shitty inner clearance when you go aftermarket coilovers, how much depends on each coilover, but generally the safe point seems to be around 9" +20 where it should clear most coilovers. unfortunately this will push out pretty far in ur guards, especially if u run a 255 down the track. if you want more carefree fitment, u could get away with something like a 8" +30.
u can then go a 9" +30 on the rear and run a 235 there as well if u want, generally when the car is running close to stock power, i find a neutral tyre setup works well on these cars as they do have a bit of understeer from factory, and not heaps of power to put down. but depends on the suspension setup etc, and how much power it's letting u put down.