What do you mean by the sentence "enter a position for 1 lot in EURUSD that trade would double the size of the account"? Your account size is measured by the NLV, net liquidation value. That value does not suddenly double if you take on a larger position size.
The way how you have worded your question make me guess that you are familiar with operating a so-called hedging account. That is not how IBKR operates. Your IBKR account is a so-called netting account. This difference makes that you have to look differently at account value, position size, and opening, modifying and closing positions.
