What does the message "Order size exceeds fund's notify amount" mean?

While mutual fund opening orders can only be entered in whole dollar amounts, any sell order including closing orders must be entered in share units. In the case of fractional shares, closing orders may be entered in the fractional share quantity…

The maximum order size for stocks is 500,000 shares.

IBKR does not have a maximum order size for most other products, but you are limited to the amount of cash or margin in your account…

REQUEST FX LARGE SIZE ORDER FACILITY

The IdealPro Forex market center provides a Large-Size Order facility specifically intended for accounts which regularly submit orders in quantities greater than standard order maximums and are willing to trade outside the NBBO associated with the standard order minimum/maximum bands in an attempt to obtain faster fills. To be eligible for FX Large Order Size IdealPro you need to submit a written request to Client Service and that will initiate a review of the account's trading history and account equity. Please allow up to 7 days for the review to be completed…

If you receive an order reject for a “Large Size” order you will need to recreate a new order for a smaller amount. Due to regulatory obligations, and in an effort to prevent sending orders to an exchange that would be disruptive to the marketplace, IBKR has implemented filters for "Large Sized" orders. These filters are based on the Average Daily Volume from the reporting exchanges. If your order is affected then the reject message will recommend a smaller amount for a new order that would fall within the "Large Size" order check parameters.     …