If you go on autotrader, select an A2, and then click on the tab for dimensions, it gives you most figures. Useful for comparisons to other cars too.
The A2 boot is 390 litres & 515Kgs load
Audi A3 is 350 ltrs & 560kgs load
Golf Mk5 is 350ltrs & 551kgs load
Astra is 350 & 525kgs load.
Golf Mk4 is 330 ltrs & 406kgs load
BMW Mini is 160ltrs & 450kgs load
Nick