I think at the moment there is some good commands but there could always be more!

Gamerule to set size of all newly spawned mobs to a certain size.

Command to change the current players size
Example:
1 - 0.2. First time using command
0.8 - 0.2 Second time using command
0.6 - 0.2 Third time and so on

Welp, this is a two-part issue (don't do that, open two separate ones) and one part is done - the server command is now much more useful

Hhhmmm.... 🤔 I have thought of this myself

That's cool that we have the same ideas. I can't wait till the 1.11.2 version is bug fixed and hopefully with new content.

ok so there is an absolute /setsizeof command and you want relative, got it.
Gamerule is a neat idea, added to the list too :)

"Second part" of the issue is done in the discord beta, there are gamerules now.
Will close this issue once the actual code is on GitHub finally.

Fixed in ede2651