I think the necessary bed-size totally depends on what you're used to and how much space you personally need when you sleep. Before DH and I got married, neither of us had slept on anything bigger than a twin, so our brand new queen size bed felt huge to both of us. We've co-slept with both our girls in our queen. When visiting family we've even been known to co-sleep with one baby and both of us in a twin size bed. (That was quite cramped, but we don't generally roll around a lot while we sleep so it really wasn't that bad.)
My sister on the other hand, got a queen size bed to herself after I moved out of my parents bed. When she moved out on her own, she bought a queen size bed so she wouldn't have to go back to a twin. When she got married, her in-laws bought them a king-size bed and they still have trouble fitting the two of them in the bed. I'm sure that when they finally have kids, the only way they'd consider co-sleeping is with a crib side-car, because the three of them wouldn't "fit" in the king-size bed.
We won't be replacing our queen bed until we buy a home (which probably won't be for another two or three years). By then, our queen bed will probably go to our girls so they can share-sleep on it. That way we won't have to buy them twin beds for a while.

Emily
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