I want to buy an after market head light len, so what size should I look for
Any of the Model T parts dealers will have the right lens for 1924. Start with Bob Bergstadt. He's having a big sale right now. If he's out, then Chaffin's or Lang's or any of the others should have it.
Not the point I do not want new one I want to buy an old one what size if I go to a swap meet
Same size as a new one. Eight inch.
The aftermarket lenses marketed for Ford are usually 8 1/8.
I have seen everything from 7 7/8 to 8 1/8 fit in a T rim. All T rims from 1915 through 1927 are the same size. The actual outer diameter of the lens is 8 inches at the clips that hold them in place (I have spread them a bit for something a bit larger). The inner part of the headlamp rim varies a bit from rim to rim and early to late, however all are close to about 7 3/16 inch (slightly under 7 1/4 inch).
Matter of fact, my "well accessorized" coupe has a pair (?) of after-market lenses in it. One was in it when I bought the car. The other I bought off of eBad in an effort to match it. They are 1/8 inch different in size. I don't think anyone would notice it if I would just stop pointing it out. They both seem to fit and work just fine.
Drive carefully, and enjoy, W2