I thought of making it around 250 mm since the steel I have is 500 mm and that would give me a spare platen. If there's a possible gain of going longer than 250 mm I'm all ears. The grinders I've used had all very different designs so I don't have first hand experience with the KMG clone I'm building.
The vast majority of grinding will be over a small area of the platen. I think this is the main reason people stick pyroceram fireplace glass to the steel plate to prevent wear. The length of the platen might help flatening the edge before grinding or flatening the spine. Maybe it could also be helpful for distal taper. It really depends on what other equipment you have for these type of things.
I forgot to mention that platens cause significant drag, so the longer the more power you need.
I don't use KMG measurements, I'm using a KMG inspired plan from British Blades (for 1600-2000 mm belts) that I fit for my needs. Power will be 1.5 KW with VFD.If you are using actual KMG measurements or parts, then I'm pretty sure that you want the 8 inch platen. Also remember to make the tooling arm longer than the regular KMG one. Your 2000mm belts are about 10% longer than the 72 inch belts that the American machine was designed for.
You can also get creative and do a liquid cooled one...SOund like you need to Butch lol. I will do a drawing in a few minutes and post it...I am doing this on mine...Just need a small water tank,an aquarium pump, a few fittings and some rubber hose.