1st: Login to Minecraft:
2nd: Load Minecraft:
3rd: Load a world with cheats enabled
4th: Use this command: /summon minecraft:armor_stand ~ ~2 ~ {Attributes:[

{Name:generic.scale,Base:5}

]}
5th: Armor Stand will be scaled up
6th: do it with other mobs: They all work except a few
7th: Use this command: /summon minecraft:armor_stand ~ ~2 ~ {Attributes:[

{Name:generic.scale,Base:5}

]} Minecart summoned but it's scale has not been increased. 

Change to boat, boat with chest, minecart with chest, etc, those will not be scaled up or down. 

Solution: Allow those listed above to be scaled

Update: Enderdragon Does Not Scale Up or Down with command

 

EDIT: Those saying it's not a bug but a feature request, it was already requested. 

https://feedback.minecraft.net/hc/en-us/community/posts/360074839111--size-command

Meaning that if you implement it in any way even if it's just attributes, it should effect all mobs, including entities.