You can fit whatever size tyres you want...
Eg. you want 245/40/18s, the middle number, 40 has to be the same FRONT AND REAR.
Otherwise you f**k up your Centre Transfer Case.
I have got,
FRONT
235/45/17R
REAR
245/45/17R
1st mesurement is for the width of the tyre.
2nd mesurement is for the height of the tyre.
3rd mesurement is for the wheel size.