The trade volume depends upon the remaining balance. I always choose the minimum lot size so that i could avoid from getting margin call (finishing up account). I choose the lot size by dividing the balance by 400, while trading on the less volatile pairs. When i wanna trade on more volatile pair, i divide balance by 800 and when i wanna make a trade on Gold. I divide balance by 1500 (for short). So, Now I have mentioned everything, you can also follow me.