It's London and space is a premium - so as above always book the room you'll want to spend 1 day in together rather than expecting an upgrade. The Bankside does have spacious rooms, especially in the lower categories compared to others and this property has probably the best lounge in London. The new Canopy in the city also has quite generously proportioned rooms but no lounge. The Conrad has some very small rooms, and some very large ones due to it's conversion in an old building, it doesn't have a lounge though. Hart in Shoreditch I was assigned a huge room, but again no lounge. Standard rooms in the Canary Wharf Hilton are rather large, and it has a lounge - its location further out and dated decor would put me off here though. The Waldorf has a lounge, but the rooms are small in lower categories due to the historic building.
The most generous with upgrades has been the Bankside for me, followed by the Hart, Conrad, Canopy.
I would advise to use the online check in to see floor plans and book rooms, with this you can normally find larger rooms within the category.