On the broker level, lot size limitations are based on the leverage and account equity available upon placing the trade.
To calculate the max number of lots you can open based on the leverage of the symbol, you will use this calculation:
(Account simulated equity x Leverage = Available Capital), then (Available Capital / Contract Size = Max lots).
For a standard lot size (1 lot) of 100,000 units, the pip value is $10 (100,000 x 0.0001).
Examples:
How to calculate max lot size on XAUUSD (Gold)
Your max trading size for 1:20 leverage on $10,000 account = Account size x Leverage =$10000 x 20 = $200,000
1 Lot XAUUSD value = 100 x XAUUSD Price = 100 x $2030 = $203,000
Max lot size = Max. Trading size / 1 Lot XAUUSD value = $200,000 / $203,000 = 0.98 Lot
Max lot size on XAUUSD on a $10k Account is 0.98 Lot
How to calculate max lot size on EUR/USD
Open a 0.01 lot size on EURUSD you will see it has a margin of $10.89
Account size divided by 10.89
$10000/10.89=918 (numbers of 0.01 lots you can open in $10k account)
918 X 0.01 = 9.18 lots (max lots size going full margin on EURUSD in a 10k account is 9.18 lots)