DH, DS and I slept in a full size bed until recently, that was a bit cramped (DS and I are small, but DH is about 280). We just got a queen, and it's plenty now. A king will not even fit in our room!
we make a queen work but I'd kill for a king, largely because dh is a bedhog and we're both trapped on our sides. a king won't fit up the stairs though. I am considering getting a twin like you described to add some more bed space - they make gap fillers for the crack btwn mattresses btw.
the cosleeper was nice in theory but we never used it. ds sleeps in bed next to me and the cosleeper holds water, my light, diapers, etc.
In case anyone needs the info... two twins side by side are essentially a standard king (the sheets will fit). You can strap the twins together (on your own or there are strap/crack filling products available though these tend to be "as seen on tv" type products).
We actually did this for a while after dd2 arrived but didn't find the extra space worthwhile. And when we moved the side by side twins didn't fit the space anyway. But it's an option...
I haven't read all the responses but we co-sleep in a queen and I don't like it. Don't get me wrong, I love co-sleeping but the queen is just too small and I always find myself smashed up against the wall. DH and DD take up a ton of space (picture snow angel) and I would love to have room to stretch out. I want a CALIFORNIA KING.
we have a king now but we co-slept mostly full-time, in a queen, till DD was nearly 2, and it was pretty much fine. we got a king mostly because i thought our current mattress wasn't that comfy, and if we were going to get a new one, i figured why not a king?
if nothing else you could sidecar a crib, or a twin mattress, if you find you need more room.