Queen size for me and my boy-toy and whatever cats choose to join us. We used to have a king size, but I am a snuggly sleeper and he seemed ridiculously far away. I do think we need to buy a king-size comforter, though–I like to cocoon myself up (habit retained from my single days) and sometimes end up stealing all the covers from him. Oops.
Twin.
I’m 5’7" and it’s big enough for me and my 5 pound cat.
I’ve always wondered why single people my height feel the need to get a full-sized bed. In my best friend’s case, she has a herd of 2 dogs and 6 cats, but that’s an exceptional case.
Cal King. I get about a 6" strip on the western edge. I actually can’t “roll over”, but have to rotate on my central axis if I want to face a different directiion. :rolleyes:
Queen. Shared with my wife. We’d love a bigger bed, but we live in a NYC apartment. Anything bigger wouldn’t leave room to walk around the bed.
Queen. It’s not quite big enough for me, but my room’s not quite big enough for a king (in addition to the the other stuff in there, anyway). Frankly the queen takes up too much room, but no way can I go smaller than that for more than a few nights.
Good question, since it’s always just me only. I had a full for years and moved up to a queen when I moved into my current housing. A queen seemed like a nice luxury and since I was buying a “real” bed for the first time, it seemed worth it to upgrade to a more sprawling size. It is kind of dumb that I haven’t grown into spreading out into the middle of it.
Queen sized. Normal occupants are myself, my wife, and our cat. Frankly, we’d love to have a king-sized bed (my wife is on the reubenesque side, and our experiences with kings at hotels suggests it’d yield more restful sleep), but we have an older house, and the bedroom simply isn’t very big. We could probably get a king to fit in the room, but it would preclude any other furniture in the room. 
King, and I sleep alone on about a quarter of it. Mondays (sheet changing days) are always fun, because I have to move a week’s worth of books and magazines and such from the other parts, sort through them, and put back the ones I’m not done with yet.
My husband sleeps on a full bed, in another bedroom, with the cats. We used to sleep together, but as the years rolled on, I needed a firmer mattress, and he only likes soft mattresses. We bought the king bed when we needed a new mattress, and he didn’t know that he didn’t like firm mattresses. Since we really can’t fit two beds into one room, we bought another softer full mattress set, and he moved into another room. We both sleep better, because our sleep hours vary quite a lot, and I like a night light and he likes it totally dark, I like to have music playing and he likes complete silence, I prefer a warmer room and he prefers a cooler one, etc.
Twin. I have considered going smaller, even though I’m obese & slightly taller than average. Beds take up room.
We’ve got a king-size bed. Just my husband and I sleep in it now, though our kids occasionally slept with us when they were smaller. They still do sometimes during thunderstorms (they’re 3 and almost 7).
Ours is somewhere between a full and a queen size. We’ve got an antique Chinese opium bed* that we sleep in, and the sizing is just big enough for us to use a queen size memory foam mattress topper and a futon mattress as our “mattress” in the thing. Too expensive to do a custom mattress, and the thick (3") foam mattress topper helps a ton in making it cozy.
*Ours looks a little like this one: same basic shape and structure, but with different details.
Twin size (the regular small size), and I sleep alone (thank goodness 
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I have a full-size bed and mattress. I sleep on one side. My cat sleeps on the other side. He is a big cat who does not like to be disturbed, so he needs a decent amount of space.
Just once I would like to be able to lay in the center of the bed. I bet it would feel weird though.
That is really cool.
We have a queen, just for me and my wife. It’s fine most of the time. I’ve often wished that there was an option between a queen (60" wide) and a king (76" wide). Sixteen inches? Sheesh. Something about 68" would be perfect.
When we stay in a hotel I kind of like the size of the king, even though it seems like a journey to meet in the middle for sexytimes. But she thinks a king would be too big for our bedroom, and as usual, she’s right. It would fit but aesthetically it wouldn’t look right.
King.
We had a Queen for a while, and it was fine, but I’m used to the King now, so I don’t want to go back. Makes sleeping with two much more comfortable, and you can spread out essentially as much as you want without bothering the other person.
I’m a big guy, so a full or smaller is incredibly uncomfortable for me. The way I sleep, my feet stick off the edge in anything smaller than a Queen, and I’m not the tiniest guy width wise either, so my wife and I together in a full is just a recipe for a sleepless night. We sometimes have to sleep in a full at my in-laws, and it sucks.
Full sized, two of us with occasional feline visits. The bed frame is adjustable for either queen or full so I suppose someday we might buy a bigger box springs and mattress, but we like the closeness. Hotels with king-sized beds feel odd to us.
Me and my kitty on my queen. I may upgrade to a King at some point after I get out of NYC; right now, it’s not an option. Kitty, at some point during the last two or three years, decided that she needs to sleep at the head of the bed, so she gets to cover the corners with kitty fur. 
At least she’s mostly gotten over needing to be on whichever side I’m facing–if I turned over, she’d actually get up and move a few minutes later.
King for my husband, myself and two boxers (of the canine variety).
Still by morning one or the other of the dogs has nudged me over far enough that one leg is dangling off the edge. They don’t actually want me out of the bed they just want to be socloseican’tbreathe.
6.5ft by 4ft.