Exhaust size catback

Gonna be running a gtb2260 3" downpipe what size catback system would be OK found a 2.25 would this be ok I know it's gonna step down and restrict flow a little

2.5" is more than adaquite and no issues with flow, heck you could run 2.00 if you wanted, stock is what, 1.75 ish. 3.0 helps a tiny bit with EGTs and sounds epic and if you do a sharp neck down you will have smoke rings up to 15mph, LOTS OF THEM if you dont run a cat.
make sure you get a high flow rated DIESEL cat, vs a aftermarket POS cat that will cause all kinds of issues.

a4 golf stock is 2.25" ish, and the muffler supposedly isn't really a restriction over the pipe, dunno bout skodas but you can measure yourself pretty easy

if going aftermarket go 2.5 all the way, 3" down pipe just makes it harder to work on and weighs more