Positions on CEX.IO are opened and closed with market orders. This means that the size of your position will simply be taken from the best offer in the order book. If, during position placement, someone has made a similar order, the best offer could not be enough to open the position. It will be used up and the rest will be covered by second best offer, and so on. This is called price slippage. The same thing happens at position closing — a market order for the position amount is placed and matched with the best offers in order book.