I want to remove my 21" front wheel and fit a 16" wheel so what or how much over do my fork legs need to be 

Thanks mate you're a legend 👍

Also depends on what bike, what's the aspect ratio of the before and after tyres and how much variation on how the handling and stability feel you're prepared to put up with.

Thats a big drop in front wheel diameter. 

Not as crazy a drop as you might think. 1.5in difference in height, split it for axle height and its .75in lower.