Hum,
We've just got a 5m x 3m cabin 2.5m high, which is as large as you'll be allowed close to the boundary without planning permission.
Children/baby would be fine or 2/3rds shots as you start running out of headroom. I've large doors and a window at one end so I can open them and shoot through if needed, plus two large side windows for natural light.
Mines fully insulated, double glazed with radiant panel heaters, over spec'd for the size of cabin but it warms up really quick when required. These type of heaters are great as they are small and don't get very hot to touch, just warm to very warm, so there's no burn hazard you get with some heaters. The insulated option is brilliant and I'd recommend it all the time. We can turn the heaters off and it doesn't get really cold and warms up very quickly.
For lighting, I've run a strip down the middle with 6 x 350 lumens led spot lighting, equivalent to 55w GU10 bulbs but only 6w. This is backed up with flashguns, brollys etc but room is a little tight with only 3m wide. I have access to a bowens 3 x 500w head setup if I need more.
The backdrop at the moment is a paper roll, but at some point I'll buy a highlight type.
At the moment it's been taken over as a teenage den but it doesn't take long to get it back into a studio.
Have a look at these people. This is ours:
http://dunsterhouse.co.uk/garden-of...x-5m-sunlight/3m-x-5m-radley-sunlight-office/
Total cost is about £8k for the fully insulated, installed, price. We had the pad system floor to further insulate it.
Decking starts going in around it this weekend.