my DS's room (he's 2.5) is about 1.9 x 2.75m and its no problem. He has a toddler bed, a chest of drawers, a chair and a shelving unit. There is still a decent amount of floor space. However I think that come the time he needs a single bed a cabin bed would have to be the way to go.
well i'd say that the bed goes down one side of the room, and the wardrobe and chest of drawers could go along the other wall. surely the wardrobe can't be much more than 2ft depth at most (i'm crap at spatial stuff) so that still gives you a 3ft gap between bed and furniture to walk between which is ample, isn't it?
my dd2 bedroom is tiny she has a single divan bed (use the underneath as storage for toys/shoes and then has small single wardrobe (although we have taken doors off and put fabric curtains on it so it can fit at the end of her bed (couldnt get doors open otherwise!) she has chest of drawers and very limited floor space, however, we have a playroom so most toys are in there and my plan is as they get older I'll convert that into a second lounge for them to have friends in (ie settee, tv/dvd player/ computer etc) so won't really need to use her room much iygwim
Ds bedroom when we first moved was only about 7ft square - he had a fullsize single bed, a chest of drawers with shelves over and a bookcase in that space. It helped that the radiator was on the bit of wall that the door opened onto, so couldn't have put furniture there anyway. And we moved the lightswitch to outside the door so we could have the shelves.
It was fine when he was smaller and didn't need a wardrobe, but we had to move the rooms round when he got bigger.