You can buy or sell different amounts of each currency pair - from as low as .01 lots to 5 (the maximum position size in the largest account) at any point. This is automatically calculated on your dashboard here.
To calculate the lot size of your trades, we add up the number of lots traded by the lot size. For example, if you were to trade four separate .5 standard lot trades, that would equal 2 standard lots (.5 x 4). Likewise, you could trade 500 mini lots (.1 x 500) to equal 50 standard lots.