Generally the ideal way to fit a golfer for iron length,   is to find longest shaft that he can consistently strike in the centre.  ( yes i know hand to floor meausurement plays a part to, but thats done first not lastl

So a good fitter will have several clubs made up to varying lengths plus impact decals.

What club is normally used for this, like a 6 iron?

But then if we do that,  but then 3-5 iron will progressively be too long for me, or am i missing something.