|Twin size plans||39"x75"||86cm x 190cm|
|Double size plans||54"x75"||137cm x 190 cm|
|Queen size plans||60"x80"||152cm x 203cm|
|King size plans||76"x80"||193cm x 203cm|
If you are 5'9" or taller, the extra length of the queen and king bed sizes gives you more room to stretch out. If you are 5'6" or shorter, the extra length doesn't make much difference.
A king size mattress is almost the width to two twin size mattresses side by side.
There is only 6" of width difference between double and queen size beds, but the 6" make a big difference. 54" for two people is just a little bit too narrow. Having a bed frame with rails that come up to almost the level of the mattress also helps, because there is less risk of just rolling off the squishy edge of the mattress as you get near the edge. It does however make changing the sheets more difficult.
More on bed plans for different sizes
on my Woodworking website