You can trade with $5, depending on your leverage. 

For example:

You have 1:500 leverage. 

You could buy 0.02 lots of GBPUSD at 1.34561 and the trade would cost $5.38. Whereas if you did the same with a leverage of 1:100 it would cost $26.91.

You would be limited in the pairs/instruments that you could trade with since some pairs/instruments have a higher contract size or lower leverage. Indices and commodities have lower leverage so you would need more money to open a position.