Brolly sheets - what size to buy?

[Missnearlyvintage]

Hi,
My nearly 6 year old DS is bed wetting (through pull ups!) every night pretty much. We've been referred by the school nurse to a continence workshop for parents/carers, but aside from that I'm just trying to get him as comfortable as possible at night, and sort the logistics of lots of wet laundry!
I'm struggling to get the waterproof mattress protectors we currently have dry after washing as they don't tumble dry, and have found brolly sheets online which look promising.
Apparently they go in the tumble dryer so that will be a big plus, and hopefully they'll mean that DS can just take off said brolly sheet on his own if it's wet, and climb back into a dry bed if he wants to, which he is excited about, (he doesn't always like to come through and tell us so he will sleep on top of his duvet, put a pillow on wet patch etc. We have tried to encourage him to always tell us and we try not to tell him off at all!).
I'm just about to order one to try but wonder what size I should buy? His bed is a single, but we often visit relatives and then he'll share a double bed - the same as if we stay at a hotel. So I wonder whether a double one will be far too big? Or if we'll get away with it on his single bed, and then it'll be handy for when we go away too?
If anyone has any thoughts, or could suggest another mattress protector type thing that will do a brolly sheets job and go in the tumble dryer I'd be very grateful! Thanks!

[CatWithKittens]

Have you looked at pjama.co.uk? We have used these recently for a 12 year old DS who still has the, now, very occasional wet bed and was very worried about sleepovers after a wet bed a couple of years ago . (Very discreetly dealt with by friend's pre-warned Mum but it still worried him.) We also use them on holiday and if we had a bigger problem would use them at home. We got the starter kit which has two pairs of pants so he can change if he wakes wet . He could then slip the wet pants into the bag which is part of the kit. Result has been that he has gone on sleepovers and not been wet, perhaps because he's not worried about it all. On holiday we've had a couple of accidents but the pants contained everything and, although we had put a waterproof sheet on the bed just in case, it wasn't needed.