IB's listed maximums are for a single order...so yes you can enter large positions.
If you are trading such size then you can easily afford a good front-end like Buttontrader...which allows you to enter how ever big of size you want and it will automatically break the order down into smaller orders that execute instantly.
ie enter a 20 million buy for AUD, will split the order into 8 mill, 8 mill, and 4 million and execute them all (assuming the offer is there to fill this)