I have no idea about the dimensions... but I do think I work with a lot more space than other people do. I'll just throw this in here as somebody who does train inside a majority of the time.
I have a long hallway that forms a perfect T section with the kitchen on one side and the rest of the foyer on the other. That's where we do most of our "heeling homework". If you are just working on training attention, rapid sits, wrap about turns, footwork... this is enough space. Same thing with Go-outs since it's a controlled situation (dog has to go straight out to the end of the hall). We also taught recalls and retrieves for the same reason - only one direction for the dog to go, control, etc...
Our living room and family room are other options for stays. And jumps in the living room since it has the thickest carpet (the family room does too, but it's smaller and more cluttered). Both rooms are also wide enough for me to train finishes there without my guy bumping into walls (like in the hallway), plus carpeted so his feet won't be slipping out under him when he does a flip finish.
Our basement rec room is about double the size of our living room - but it's all tile floor, so I just work on dumbbell training and heeling/f8s down there. No jumping.
If there isn't a lot of snow out there (like now), we train out on the road in front of our house.
If there's snow, I generally back the cars out of the garage and do some training out there. It's a two car garage, so plenty of room.
I also have a couple favorite spots downtown which are sheltered from the weather (covered pavilion, covered storefront) that work out great for training full distance stays and recalls.
*** If I were training in tight quarters, I would focus on attention, pivots, side steps, scoot heeling, scoot fronts, dumbbell work (it's not all about long distance retrieves), stays... basically everything.
My rally instructor who also teaches comp obedience and was an assistant in our obedience classes for Jacks first couple years - she calls the foundation exercises (pivots, footwork, heeling with attention, scoots, etc) kitchen work. I get the idea that the majority of training that she does with her dogs away from the training building happens in her kitchen.