Are you wanting it to wrap to a second line (which looks kinda cruddy)? Otherwise increase the width of your block to fit the maximum number of characters you allow (and if you allow 20-25 then you gonna have a BIG block).
You are looking at at least 140px width for that with that username to be on one line AND you will also have to edit the message block to make adjustments.
Depends on whether you want to expand the block, shrink the font size or fit in current area. If the latter, then yes - you will need to figure out how many characters. On one of my forums it's 15, on another 18.