Hi
The width of the mattresses is approximately the same as a small double - also known as the 'bachelor occasional' - but the mattresses in the California are a bit longer so 'long small double' size sheets and protectors fit really well as supplied by 'the linen cupboard'. I bought the cotton percale protector and cotton sheet for the top and leave on all the time. If putting roof down damp then I put a survival foil sheet over them to keep it all dry.
John Lewis do a small double fitted sheet normal length which I bought for the bottom mattress and it fits well on the width, but one corner of the comfort mattress curls up a little so the long small double would fit better but is more expensive. I leave the mattress protector on with the sheet on the lower mattress too, and cover with a large fleece blanket when it is folded back into the shelf in the boot, so it's easy to pull over if needed. Also I keep four pillows, blankets, ample double duvet etc stuffed in the wardrobe and find this all works really well. Hope this is helpful!