You can set the workspace to any size you want, but 300x300 will be pretty close to the lens's limit.
You may know this, but just in case- the basis for the numbers 420mm and 300mm: f=420mm, that will be the lens's approximate maximum drawing circle, by design. So, if you draw a 300mm square and a 420mm circle, you'll find the square will almost fit perfectly within the square
There's a math computation to figure this but I don't know what it is
-- but if you know only the f-number of a lens, you can always figure out the approx. work area just drawing a circle and a box!
Having a 300mm lens, you're not going to have a lot of power, you should consider getting a 150mm (about an f-210mm) lens.
When you save anything to the F3 settings, YES EzCad saves all the parameters. But, those parameters won't transfer to other size lenses. I have 3 lenses I use on my Triumph, and I have 3 EzCad programs in 3 different folders, for which lens I'm using. I just added "100", "150" and "220" to the name of the .exe file that starts each EzCad, then created a shortcut for each, so I have 3 EzCad icons cons on my task bar to start the right program for lens I'm using...