Interactive Brokers has a house limit for portfolio margin users in which your gross position value (GPV) cannot be more than 30 times your net liquidation value (NLV) when opening a position. "https://www.interactivebrokers.com.au/en/trading/marginRequirements/marginCalculationsSecurities.php" I'm wondering if any brokers allow a higher GPV/NLV ratio limit? Also, If you know the arb strat I'm pursuing and have been down this road yourself, I'd love to know how it worked out for you.
Besides this limit, most clearing brokers provide a credit limit and a debit limit. They do not want an account of $500,000 carrying a credit of $50mm. In addition, there is a regulatory limit for how much one clearing broker can lend. They are not going to allow one customer with $500,000 to borrow $50mm either. This has nothing to do with risk.