Good question.
First of the best rules I found (haven't played a siege yet) are from Warmuster Magazine. They still need tweaking though.
They might however inspire you.
I bought my castle to stretch across 6x4 and still have left over walls for the three keeps inside where a city will reside too.
I think you have to think about army size. How many units and bases? One unit or 2 artillery units fits a tower. Double that in a keep. Normally a unit only has one rank on the walls. A warmaster base is 4 cm. Kallistra walls are 5, 10, 20 cm. Smaller wall sections are an advantage for when walls collapse. At least keep a mix. When dedicind also think cavalry and other units doesn't occupy the walls and towers. Say 30-40 %. Finally a 1/4 or 1/3 of the army may be on reserve/relieve force duty if you want to play that kind of game where they randomly decide if arrive in later turns to relief the Besieged.
By the way maybe think 5cm wall for each stand to make space.
Doing this math should allow you to calculate your castle footprint quite adequately.
Also remember that going small is fine. Get some experience and the expand. I just got a big one as I wanted huce ass games....
