I have just been through this. I went to Mattress Firm and an extremely good sales associate was very helpful. I was getting ready for hip surgery and couldn’t get on my old mattress because it was too high. Plus, it didn’t support my lower back, so it was impossible to sleep. I picked a new mattress and it was delivered. It, too, was too high. I guess the platform it was on at the store was lower than my own, AND it was too short, even though I had measured the length of my old one. Apparently, the way I measured was different than the way the manufacturer does. Aagh. Anyway, I talked to my sales guy and he found a 12 inch mattress for me and had it delivered. Well, I could get on it, but it was as hard as a rock. Meanwhile, I had my hip replaced and realized that the hard mattress had to go back. This time I had to pay a $150 pick up and delivery charge, but was able to exchange the mattress. My caregiver brought me in to the store and I tried mattresses again ( I do know that this is getting old) and ended up with a Sterns and Foster Elite. It is incredibly comfortable and supportive, and my new hip allows me to hop on. So all is well, and I hope there are some lessons for you in my crazy tale!