I can't see why you think that "the English part of the senate will always win" Jen. We have a UK parliament at the moment - well two really - the Commons and the Lords. A Senate would work on the same party basis as far as I can see.
I don't think it is as complicated as you think whitewave. The Senate would be the revising chamber for the countries parliaments, just as the Lords is now for the Commons. It would also retain some of the cross country powers that are not devolved to the countries. I wonder if many of us know that the Leader of the House of Lords is Baroness Stowell of Beeston; I certainly don't know who the leader of the US Senate is. So yes, in the same way that the Commons is the only English parliament currently, there would be a PM for the English Parliament although First Minister may be preferred. We have a First Minister for Scotland, Northern Ireland and Wales but England may wish to continue with Prime Minister.
As for numbers, I don't know. I have only just started looking at this because various cross party MPs have mentioned it. There are currently 650 members of the commons so I would like that halved but I have really no idea. Better people than me could no doubt tell you. Without the Lords and with fewer in the UK parliament there should be some large savings.