255/70R15 probably the biggest you can get without rubbing. Honestly, I'd go back with 235/75R15
You can actually do the math an measure off how big the tire will be.
255(width-mm)/70(% of width)R15(rim size)
Width Profile Rim
You have a 205/75R15, Which is 205mm wide, 154 mm tall.
If you get a 235/75R15, The tire will be 2 cm wider and 2.2 cm taller.
A 255/70R15 will be 4cm wider than stock, 2.4 cm taller.