Technically we don't have a hard lot size limit, the "size limit " depends of multiple factors like the leverage, balance of the account and price you are trying to get filled. Similar to a personal account.
Here the process to know the maximum of lots you can have open.
Calculate the margin required for your buy or sell:
Formula to calculate margin required for a buy:
Margin Required = (Contract Size / Leverage) * Ask Price
Formula to calculate margin required for a sell:
Margin Required = (Contract Size / Leverage) * Bid Price
Calculate the maximum number of lots you can buy before reaching a margin call:
Formula
Maximum Lots = Account Balance / Margin Required
Here is an example on how to calculate the maximum lots open of US30 you can buy on a $100,000 balance account with the given account details of 10:1 leverage on indices and 100% margin level, you need to consider the leverage, the ask price, and the contract size.
Calculate the margin required for one lot:
US30 bid: 34369.72 - ask: 34370.82
Margin Required = (Contract Size / Leverage) * Ask Price
Margin Required = (10 / 10) * 34369.72
Margin Required = $34369.72
Calculate the maximum number of lots you can buy before reaching a margin call:
Maximum Lots = Account Balance / Margin Required
Maximum Lots = $100,000 / 34369.72
Maximum Lots ≈ 2.91
So, with a $100,000 balance and 10:1 leverage on indices, you can buy a maximum of approximately 2.91 lots of US30 before reaching a margin call. Please note that this calculation assumes no other open positions and that you are using all your available margin for this trade.
Use this formula, remember to take in consideration this:
Account Balance at the moment
Leverage (See here)
Ask/Bid prices