What does "Display size is not supported for OTCBB stocks" mean?

The order rejection "Error 503" message is displayed when your account does not have trading permissions for the product you are trying to trade.

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When you see the message "No opening trades: ineligible PINK stock" it means an opening order for that contract is not allowed, even if your account has trading permissions for US Penny Stocks.

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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…

Available Size in Order Presets, Hotkeys and Buttons means the current Bid Size or Ask Size.

Additional Information…