Ok Good you know it make Sense Rodney. . . Lol

Start by giving sensible cutting area and maximum material thickness you'll cut. Just realise that the larger you go the harder and more expensive it gets.

Wouldn't advise going larger than Quarter sheet for First build on budget. Even then I'd only go with this size if really needed and would be limited to soft materials only. Ie No cutting aluminium.
This is mostly because with low budget you'll want to keep the machine restricted to using Single ballscrew on the long axis. Going wider than this or wanting to cut harder materails will require 2 ballscrews. This means other than the extra Ballscrew and bearings you'll need Extra motor, drive, coupler and larger power requirements. These bump the price quickly.

If you go with smaller cutting envalope of say 900 x 600 or 600 x 400 then you'll build much stronger machine that is very capable Cutting aluminium.