If at this stage you can't afford or justify getting a piece of aluminium big enough to cover the whole bed, or strips to cover the bed leaving slots, then I'd say the minimum is a piece wide enough to span the machine and long enough to be useful. That means about 4' by something, maybe 1200x600 would be enough to cover most things. The biggest single aluminium part I've machined on my router was 1015x700x25.4mm and the biggest aluminium 'job' so far has been 1480x715x20mm, which probably doesn't help much with your question as it depends entirely on what you're planning on making. Also these parts were machined when I still had an MDF bed, so if the part wont fit on you're aluminium bed it's not the end of the world. Since your bed will be well supported I see no reason to get more than 20mm thick aluminium.