@Commodity_Trader
You mean a market order of 7000 contracts? Sorry if this wasn't clear. When you get bigger sizes, you have to model your executions, you can't just go market orders. The way I calculate my slippage is I have a reference price generated by the trading logic (e.g. Buy market current price is 100) and then I use an execution algorithm to send a mix of limit and market orders. My slippage is then the difference in average volume-weighted price and the reference price I would have had if I was trading 1 contract at signal. Most of the execution is done on the passive / limit orders side, market orders are for when part of your clip wasn't filled and you still need to complete the order otherwise you didn't execute what the strategy generated.
How much can you execute with market order only? Probably not more than 300 contracts but that's a pointless question because with size, you WILL model your executions.
It's one day of work with tick data, the execution logic is always very simple.
Also FYI block trades that you mention are executed off exchange through OTC desks (e.g. After market closes through a prime broker) so they don't impact the market.
If you put a limit order for 1000 contracts and you get filled by takers, you will be booked with the exchange as many small trades, of whatever size each taker took.