Can I make this into a 500x1000mm machine or is it strictly sized to this spec?

In theory - the X-axis could be expanded to probably a couple meters. The problem would be the Y-axis would then be farther apart and it’s driven from only one side. This was done to decrease complexity and cost but beyond some distance (of which I’m unsure) it would require driving the Y-axis from both sides.
I’d guess you could probably increase the distance between the Y-rails to ~400mm and it would probably still work but beyond that you would need a slight redesign to add another belt and motor.