There are various methods to size orders:

a:  By default, one lot is set to 100.000 shares for forex, 1 for futures
and 100 for stocks. You can change this value for each contract.

b:  In addition you can set a general multiplier on top of a: and which   is per MetaTrader, e.g. 10.0 which means 10 times at IB.

c:  On top of a: and b: you can enable the balance IBKR / MT balance
relation. Example: On IB our balance is 100.000 at MetaTrader         50.000. So the balance relation factor is 2.

Example that summarizes all options:

a: The shares per lot factor for EUR.USD = 100.000 (default für forex)

b: The general multiplier for MetaTrader instance is set to 2.0. (default 1.0)

c: The balance relation factor is enabled and the balance at IBKR account is 100.000 and at MetaTrader 33.333. So the factor is 3.0

Note: The balance relation factor is not respecting different account currencies in IBKR and MetaTrader account. If the base currency should be different, you should work in the exchange rate in the general multiplier (b:)

Let’s place an order in MetaTrader of 1 lot.

Final IBKR ordersize=a:  x b: x c: = 100.000 x 2.0 x 3 = 600.000

Video about setting order sizing