The advice in the post above is valuable - go and see some, get an idea of floor space in the different designs.
Mine is 36 x 12 and it is fine for me, my wife, 16 year old and retriever/alsation dog. Think about it, it is larger than alot of city flats.
I do admit, it was more fine, in a space kind of a way, prior to my son choosing us in a caravan over his mum in a £700,000 mansion, but for 2 and a dog, you'd be fine.
There are a few photo's on our blog
www.axeheadfarm.co.uk Your sleep thing - you could get a tourer for sleeping in and live in the static?
Main things about a static:
You can some without under floor insulation or double glazing. Probably avoid those.
Definately chain it to the floor - we had 3 concreete plinths made with eyes, to make sure the van was horizontal, but then to chain it down.
Replace carpets with laminate.
Get a log burner
Block off the sides underneath, even with hay bales - in winter the air underneath really cools down the place.
Building a porch is amongst the next things on our list!
As with any home - it will be how you make it. :-)