Going bigger

[huck]

I have a stock 1988 ford f150 xlt lariat 5.8l with 15x8 stock rims. What is the biggest tire I can fit

 

[huck]

It's also a 4x4

 

[Skitter302]

Yo! welcome to the F Zone, feel free to introduce yourself by posting a introduction here: http://fordfzone.com/forum/18-introductions/
To answer your question; a 10.5" x 31" r15 is as big as you can go in the front and even then some tires might rub. If you do a leveling kit in the front you can fit a 10.5" x 33" r15
If you want to compare that to metric sizing try using this "Tire Size Calculator Charts by Wheel Size" https://tiresize.com/calculator/

What make and model of tire ya hoping to fit on the truck?