Don't know if you want metric or imperial, but I uploaded a zip with a full set of metric ones some years ago:
https://www.cadtutor.net/forum/topic/62352-titleblock/?tab=comments#comment-514547
1- Click "Customize"
2- "Preferences"
3- "Use Large Toolbar Buttons
4- Restart 3ds Max to see your new icon size
If "Use Large Toolbar Buttons" is checked then you already are using the largest icon size possible
Adsk