Hi, If you have the standard 90 wheels then they should be 16 inch diameter ones. You need to check the width too. Somewhere on the wheel rim should be stamped the diameter and the width, described as a J rating,ie: 16 x 7 J or something similar. Older 90 rims are quite narrow so restrict tyre size choice. Once you have the rim size then talk to a 4x4 tyre specialist about tyre sizes.
I've got 235/85 x 16's on my 90 and they are fine. 265/75 x 16's should fit without too much trouble on some rims. Don't forget to adjust steering stops to prevent tyre rub on the radius arms on full lock, and lastly, remember that if the tyres are too large your speedo will read wrong and need calibrating.
Hope this helps
Damian