Are Stock Yield Enhancement Program loans made only in increments of 100?

No. IBKR may borrow any quantity of shares from  your account. However, IBKR will not borrow fractional shares.

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What will be the impact of lending stock on the calculation of my margin and my excess liquidity?
Are shares loaned only to other IBKR clients or to other third parties?
Under what circumstances might a stock loan be terminated?
Are shares loaned only to other IBKR clients or to other third parties?