Changing end dinette into a permant bed space

Bev from Wirral

I have an Avondale Dart 5 berth with an end dinette and am thinking of making the end dinette into a permanant bed with a proper matress, as finding the seat cushions uncomfortable to sleep on, am looking for suggestings on  where to buy a mattress and what
size as I would like to order one asap in time of our next trip. I have tried unsucessfully to look  for the technical dimensions on line, am hoping someone can point me in the right direction to find out this info.