DS1 is currently in a small single bed so over the next year or two (he's 3.9 now) we'll look to get him a new bed and his younger brother can have the small single.
Ds1 is very lucky to have a truly ginormous bedroom which would easily hold a kingsize bed but this seems ridiculous for a young child. But if we just get him a normal single, it will look tiny in the room.
Would it be odd to get him a double at the age of 4 or 5? It would definitely look better in the room and we wouldn't have to get him another bed when he's a teen/young adult.
I had the tiniest of box rooms til I left home so this idea is alien to me :)