About to trade two players for a pick
but it'll mean I go to 55 players in roster
do I get time to choose which other pair I cut or does it automatically choose two!?!? :O

you can have 60 players until the 1st regular season game.

so i think you should be ok.

I'm on second game now though

If the trade is final then release the two you wish to be released. Otherwise the AI will release for you.

But I don't wanna release in case he doesn't agree to it? ???

It needs to be final first, so you can wait for him to agree.

But surely when he clicks agree that makes it final right? ??
I'm panicking now

If he agrees, you have until the next sim to make the necessary cuts.

OK
But I'm off to bed in an hour. Let's say he agrees in a few hours (american evening). The next sim update is at 6am here.
So I Won't have time to cut.
Should I just withdraw and agree the deal verbally?

You could always withdraw & offer again tomorrow. Let him know what'what's up.