Getting a king mattress up the stairs...

Hi skyjumber,

This would vary depending on the mattress, the type of compression, and the manufacturer. You would need to talk with the manufacturer of a specific mattress to find out the shipping dimensions of a particular mattress but I think it’s fair to say they would be “much smaller” than an uncompressed mattress.

If you look at some of the images here you will get a general sense of their size.

If you mean two twin mattresses that are separate mattresses but just placed together then most people would notice the slight gap between them if they slept in between the two mattresses. It would also depend on the specifics of the mattress and the size of the gap between them (how rounded the corners of the mattress were and how closely they fit together). There are some products (see post #2 here and post #8 here for some examples) that are made to fill in the gap them but then it could also defeat the purpose of having two mattresses that can be used independently (such as on an adjustable bed or for better motion isolation).

If you mean a mattress that has a split construction (each side of a mattress made with different firmness levels inside the same cover) and if the mattress has a suitable quilted cover or a thin layer of padding or foam above a split layer then most people wouldn’t feel the split in the foam although they would feel the shift between different firmness levels on each side (see post #2 here).