325/60/18 tires on a stock height 98 4x4 f150 xlt.

Will they rub? Will i have to change my gear ratio?

How tall are these tires then? How can I figure that out?

 

Yes they will fit, but barely. You will have to crank up your torsion bars and trim a little plastic off your front bumper's lower valance. Those tires are basically a 33x13.00-18 tire. I would suggest using a 9 to 10 inch wide wheel with 4.5 inches of backspacing.