Agree 1200x600 would be my minimum if planning on cutting thicker Aluminium has most Single parts will usually fit in that space and only if your nesting several parts will you need much above this. Even thou my 1200x1200 bed is all Ali (slots) usually half of it has piece of MDF or HDPE spoil board on it.

If your cutting thin Ali plate like 3-4mm then you'll probably want more space and be cutting large single pieces anyway. So then again you'll probably find an MDF spoil board will be used for holding and bed saving reasons.!

Part holding is a challenge and can be pain also it can be very part dependent.? You'll find even with an Ali bed you'll use some form of protective material or spoil board between part and bed.!