Is Your Feature Request Related to a Problem? Please Describe.
For some reason "slim" arm skins do not get recognized for ReSkin.

Minecraft and Mod Information

• Minecraft Version: Any
• Mod Version (if applicable): Any
• Forge/Fabric Version : Both
• Are you using other mods? [yes/no]
  • Mention any mods that you may want to explore compatibility with or integration with.

Describe the Solution You'd Like
Add another command or a command string for /setskin which changes arm size.

Describe Alternatives You've Considered

Additional Context

Use the /setmodel slim command if the model commands don't exist then you are on a version too early for that being added.

I believe in some versions you could specify the model after the URL on some versions.