I have owned 3 IKEA beds and 1 IKEA crib. I can't speak to the adjustable size ones as I've never had one, but the standard twin-sized beds are a standard twin size. We also own 4 IKEA mattresses. My DH and I switched to a SULTAN HASSELBACK for our bed in 2004 and it's the most comfortable mattress we've owned. We have the KURA and the HEMNES twin-sized beds and used to have the (now discontinued) ROBIN twin also. The ROBIN was a bit shaky, but the KURA and HEMNES have held up well. I highly recommend the IKEA beds and mattresses; I think they offer great value for transitional furniture. My parents bought my sister a set of IKEA MALM dressers when she was 12 or 13 and they are still in great shape (she's almost 20).