Total Trading Power: With 1:125 leverage, your $100,000 can control positions worth up to $12,500,000 ($100,000 * 125).
40% Margin Rule: You cannot use more than 40% of your available margin on a single trade or multiple trades in the same direction on the same instrument. This means you cannot commit more than $5,000,000 ($12,500,000 * 40%) to any one trade or set of trades on the same instrument in the same direction.
Max Lot Size for a Trade: Forex lots are typically 100,000 units of the base currency. To find the max lot size, we calculate the maximum amount you can commit ($5,000,000) and divide it by the value of one lot in your account currency. For simplicity, if we assume you're trading a pair where the base currency is the same as your account currency, the maximum lot size would be 50 lots (since one lot is $100,000 of position value, and you can commit up to $5,000,000).