The description of the paladin's smite class feature contains this sentence: "The extra damage is 2d8 for a 1st-level spell slot, plus 1d8 for each spell level higher than 1st, to a maximum of 5d8." So this gives us a 2d8 for 1st lvl slot, 3d8 for 2nd, 4d8 for 3rd, 5d8 for 4th, and 5d8 for 5th...