I have 18" wheels on my 2010 F150 and want to go smaller to save gas what size can I go down to

You mention you have 18" wheels. If you mean smaller diameter, it's not going to save fuel, on the contrary it lowers your gear ratios, including final drive. Your engine will be running faster than before at same speed. Loss of economy. Your Ford was designed as a package with fuel economy part of the package and wheel size is 18" stock and for a reason. If you meant smaller width, you can from from P265/60R18 to, say, 245/65R18 and maintain the overall diameter, and the rpm of the wheels at same speed as before, with a higher aspect ratio (the 65) to compensate but the economy change will be negligible.

My 2010 F150 came stock with 17 inch rims. What kind of mileage are you getting? Around town I get 16 to 17 mph. On road trips about 22.