You basically have 2 options to do something like that:
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You can modify the bottom frame to allow large items to be inserted, like @TysonSwan (awesome table!). You can include removable boards if you like, so you can mill small items and remove the board to mill something larger.
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You can raise the X and Y axes by getting taller plates Y axis endplates. Inventables doesn’t sell them, so you’d either need to source them or make them yourself. If it helps, I made Y axis stabilizing braces out of 1/8" aluminum on my X-Carve (Y Axis Braces and Belt Dust Shield). If I was to make plates to raise the height though, I’d make them out of steel.
If you want to make your own plates, the drawings with dimensions are available on the Inventables site. All you’d have to do is modify them and then either make your own, or take them to a machine shop to get them made. If you use steel, be sure you paint or powder coat it so it doesn’t rust away.