From TF2 Wiki
Item Tag Naming
This is a reference for all those people wondering how the Name Tag and Description Tag character's are shown.
The Name Tag will be shown like picture below if all 40 characters are used. And the Description Tag will be shown like picture below if all 80 characters are used.
Other information: You can put as many spaces as you want, but you can't start with a space.
The first 67 characters or less for the description tag will be shown on the first row. The remaining characters are shown on second row.