I have a 2003 Ford f250 5.4l and have spring a leak in my radiator around one of the transmission cooling lines. How do I determine which radiator to pick? Each Auto parts store says there's multiple to choose from with all different dimensions

Welcome to the site ..

Generally spec’n the radiator out, meaning checking physical measurements isn’t difficult. Usually it’s core thickness/ how many rows-bars, overall dimensions, and sometimes fitting orientations,size.
If your unsure, what you might do is call your local Ford Parts, give them all the trucks specs, VIN if asked, and have them not only give you a price (they can be competitive with/against a good quality aftermarket offering) but have them look up a replacement Ford unit. Using that Ford part number, sometimes makes sourcing an aftermarket easier since some vendors will list OE part numbers in the aftermarket unit descriptions/specs, as well as alternate OE numbers that the radiator may cover.
Aftermarket units vary in quality, generally following price.. Also, when considering aftermarket, look at the warranty period each vender offers.. They tend to vary with brand/vendor, and quality and price, warranty extends further...

Thank you so much that worked!