Opening and closing
Sizing, slippage, max size, and partial closes.
You size a position by what you are willing to pay, times a leverage. Slippage is a tolerance on how far the mark may move before an open is rejected, not price impact: there is no book to walk. Closes take a fraction of the position, from 25% up to Max.
Sizing
You type what you are willing to pay and choose a leverage; size is the product. Sizing by what you pay rather than by notional keeps the worst case in front of you: the number in the pay field is what this trade can cost you.
Slippage
There is no book to walk, so slippage here is not price impact. It is a tolerance: how far the mark may have moved between you pressing the button and the transaction landing. The default is 1%, with 0.1% and 0.5% presets and a custom field up to 50%. Set it too tight and a moving market rejects your open; set it too loose and you accept a worse entry than you meant to.
Closes carry a fixed 5% bound in the same direction. It is not a setting, because a close that fails on a tolerance is a position you still hold.
Max size
Every side of every market has a largest position it will take right now. It is the tightest of three limits: the pool's risk headroom, the market's open interest cap for that side, and its base exposure cap. When your order is bigger, the ticket refuses it and says what the limit is at your leverage. When it is small, the pool is busy, not broken.
Closing, in full or in part
Open the position and use the Close tab. It takes a fraction: 25%, 50%, 75% or Max. Close all of it and the position settles and disappears. Close part of it and the rest stays open at the same entry price with proportionally less collateral behind it, so its liquidation price stays where it was. Taking money off the table does not make the remainder safer, which surprises people.
The tab shows the estimated realised PnL, the closing fee and the margin freed before you sign. The closing fee is only taken out of profit; a losing close pays none.