RE: Whats the biggest tire to put on our stock rims??
Biggest size on stock 4x4 rims in a 4x4 is 30".
To answer your subject question. Any tire diameter size you want, as long as you keep the wideth to 11.5" or less. Getting them to fit the wheel wells is a different subject.
Being a '88, no doubt it came stock with 215/225. 235 is a good tire size to run. The next larger size is the 30". The key thing is the tread of the tire. I prefer to run the BFG All-Terrains. They have excelent handling in the snow, and rain, plus they do decent in the mud, and they do excellent in the sand. No tire does any good on ice, unless you can run spiked snow tires, or chains.
Going to larger tires does have its draw backs, speedo will be off, so will the odometer.
The truck will feel like it lost power.