I would say 6" but if it were me I would go with a 8" Rize kit for that truck they make a nice quality kit http://www.rizeind.com/03products/02...en_ford01.html
Taken from the Rize site
"Front Lift System 04'-06' F150 & 03'-06' Expedition
The front kit consists of bracketry that is modeled in 3D CAD using an artistic design touch, but with form and function foremost in mind. Starting with 1/4" A36 steel the bracketry is CNC cut and formed into a boxed and gusseted configuration, then welded at strategic points to maximize strength. By using these manufacturing methods and combining them with our quad link support struts and integrated skid plate, we at Rize Industries are able to supply drop down bracketry that is both stronger and more visually appealing than that of the competition.
The industry standard of raising the steering point way up on the spindle to address the steering issues on most of the competitions lift kits was not an option for Rize Industries. That method not only puts undo leverage on the rack & pinion system, it also promotes a "heavier" steering feel. Here at Rize Industries we have designed a system that we refer to as L.S.R.S. This unique "Linear Slide Rack Support" system helps isolate the rack from the exaggerated forces that are generated by using larger wheel and tire packages. Not only does our L.S.R.S. help take stress loads off the rack & pinion unit, it allows the steering point on the spindle to remain in the stock location, thereby creating a factory type steering feel.
Combining all this with special bracketry to use the stock coil-overs, or an upgrade to optional aftermarket coil-overs, what you end up is simply the finest F150/Expedition Front Lift Kit on the market, PERIOD, END OF STORY!"