Spacing block is very convenient, but leaving the user total freedom to adjust its size bothered me too.

I like the suggestion of having 3-4 predefined default sizes (e.g. small, medium, large etc) instead of leaving it entirely up to the user. Then, the themes would be able to override those values, remove them, or assign new sizes. In some ways, the functionality is similar to how thumbnail sizes work.

An approach like that would also give the Theme's developers the power to use pixels, ems, rems, vh, or whatever they like.