I never actually measured the Ikea mattresses. We knew we wouldn't be able to haul them back from Seattle to Portland on the top of our car, anyway, and well, we like futons!
As for what kind to get, we just went to a local store, talked to the people, lay down on a few futons, and picked something that felt comfy. I'm still sleeping on the queen futon I got 15 years ago, but let me tell ya, futon technology has changed since then. The kids' futons are way more comfortable than ours! They fit perfectly on the bunkbeds and we've never had a problem with them. (We have Ikea's smallest, most basic bunk bed).