If you are using metatrader, you can check the specification of the pair that you are trading in the market watch window (right-click pair and select ‘specification’). The contract specification can tell you the lot size and the margin currency. For example, the contract/lot size for S&P 500 is probably 1. The S&P 500 is around $3,100 right now, so one lot would be about $3,100 USD in value.