I assume you're talking about the sj70 listed on your profile. (Edit: maybe I should read the title properly
)215/75/r15 is the biggest legally, the biggest that fits without any other modifications as far as tyre clearance goes, and, importantly, the biggest without the need to rectify the gearing. (Although that's debatable for off-road use - i'd personally want reduction gears in a sierra regardless of tyre size for that use)
Suspension lift won't help clear tyres, although a body lift can assist in certain aspects. Spacers aren't good for much either, (apart from being illegal) they will just make fitting larger tyres more difficult, except for leaf spring clearance. (depending on the tyre)
In short, 215/75/15. Anything bigger requires work to make the tyres fit and gearing needs to be rectified to have a nice, driveable car.