No larger than anywhere else and as Britain has little to sell the traffic can be only one way due to our lack of industries - that's going to sort out our economy to no end, isn't it.
It's nonsense to suggest that Britain would cease to trade with Europe - do you honestly believe British firms would stand for such a move?
So if we have so little to sell to the EU, why do we need access to their market?
If they have so much to sell to us, they need access to our market, and, as such, access to their market would be a suitable quid pro quo.
Asking whether the British firms would stand for it is pointless. It is highly unlikely that it will ever happen because, under the same argument, the EU firms would not stand for it.