Silver Pen is a @joecooldoo's Pen Online client.
What this offers:
2. A way to change your user ID/Nickname
3. To see who's currently drawing
4. A context menu by shift-clicking
5. and, my favorite, a size slider.
Only people using Silver Pen can see different sizes, as size is not recorded in vanilla.
To people using Online Pen instead of Silver Pen, the size will be the same size, which is 5.
Oh, and this is another link