The Maximum Position Size is anywhere from 2-5 standard lots at any one time depending on the account that you have chosen. This is calculated across all pairs that you are trading. We do allow you to trade micro and mini lots. So if you were in the $300K Trading Combine you can trade up to 30 mini lots or 300 micro lots at a time.
An example: If you are trading 2 standard lots of EUR/USD and 1 standard lot of USD/JPY at the same time you would not be able to open up any more trades in the $300K Trading Combine as that would exceed the 3 standard lots allowed. If you do open another lot that would be in violation of the rule and the account will need to be reset.
You are solely responsible for ensuring that you do not trade more than the maximum standard lots (or the equivalent) as TopstepFX MetaTrader 4 may not reject orders that exceed the maximum position size. Please make sure to include any outstanding orders that may be hit when calculating your position size.
You can see what your maximum position size is here.