What are the Parameters, diameters, or given dimensional rules for a standard christian cross?

I am about to produce a celtic knotwork illistration and the work is to be based around a christian cross image, so I need to know if there are any "correct" definitions of the cross before I draw it and commit several days of work to this, the picture is for a heavily devout english christian of the C of E . Any "given" rules would be welcome as I am having a great deal of trouble getting any definative answers elsewhere.