At what point..

[snakey68]

Does a young blood go from liking to be in a small secure enclosure to something more spacious that allows them to stretch out?

its not so much a case of liking to be in a small enclosure basically you want any blood at any size to be able to stretch out their length its just that as a hatchling they don't require much length, they are thick rather than long and feel more secure with less floor space.

step up as required as the animal grows imo any snake should be able to stretch its full length in any enclosure it is captive in. Minimum 3/4 of the animals length as far of length of the tank or tub. Obviously with hides and water bowl etc the space is quickly taken up so bare that in mind as well.

Space should be enough to allow temperature gradient to be maintained. You want provide room for the animal to move around etc Bloods are generally lazy snakes and don't move around much but I still think you should be more generous as they grow but that's just my opinion.

They should be able to comfortably move from the warm end to the cool end with little trouble.