How big can i go?
 

I have a stock 98 f150 and i dont have a lift or anything. I want to know how big of tires i can put on there with out running into clearance or turning problems. I am buy some new rims and i dont know the backspacing and i think that they are 8 inch's wide. I dont mind doing any triming work on the bumper i just dont want to put a lift in.

If I'm not mistaken, you can get a 16x8 with 4.5" BS (+12mm offset) and fit a 305/70-16 (or a similar size 17" wheel/tire combo) without a lift or level. That's what I ran on my last Supercrew. No rubbing or trimming required. (That is assuming you have a 4x4).

does anyone know how to measure the Back Spacing on the rim. Its just the depth from the lip to the bolt hole pattern, right?

Yes, you can fit a 305/70-16 (33x12.50) on an 8 or 10 inch wide wheel with 4.5 inches of backspacing. With a 10 inch wide wheel you would have to trim a little plastic off the front bumper's valance. This is assuming you have a 4x4. If you have a 4x2, I'd stick with a 265/75-16 (32x10.50). Backspacing is the distance from the mounting surface of the wheel (where it bolts to the hub) to the edge of the rims rear lip.