MEHT
 

If MEHT is quoted as 60 feet for 3 degree PAPIs, what exactly does that mean and what are the implications for different sizes of aircraft?

I'm sure I should know but don't.

With wheels below the eye height, the wheel height at threshold will be less than 60 ft and vary depending on aircraft size and approach attitude.
PAPI is normally harmonised to value consistent with the majority types of aircraft using that runway.
For specific long body / large aircraft, a second set of PAPIs may be installed further along the runway to provide an increased wheel height at threshold.

Quite significant in a 747. IIRC, at 3 degrees and a touchdown point of 1000' in the runway, the TCH is as little as 4'. The 747 is the reason most touchdown points were chnged to 1500' from 1000' in the 1970s