How much bigger will my tires be?

I currently have 255/65/16 size tires on my 1999 ford f150 and i'm upgrading to 245/75/16. How much bigger will they be?

The 255 is the width in millimeters, 16 is the rim diameter in inches (yes, inches, I know right?), and the 65 is the aspect (height over width).

Your old tires were 255/65, meaning they have a section height of 166 mm. The new tires are 245/75, so they will have a skinnier, but taller section (184 mm). Stood side-by-side, the new tires will be 36 mm, or about an inch and a half taller (because the section height is doubled).