How to set default size for characters in threads !?

All the font sizes are located in the Main CSS page. Assuming you mean the post font, you might take a look at changing the font size in the body block (first one on the screen).
 
Go to your Admin Control Panel > Products & Plugins > Add new plugin :
1.Product = vbulletin
2.Hook Location = postbit_display_complete
3.Title = Change Message Font
4.Execution Order = 5
5.Plugin PHP Code:
$post['message']="<font size='3'>".$post['message']."</font>";
6.Plugin is active = Yes.
 
If you meant the amount of characters displayed in a thread go to

ADMIN CP > Message Posting and Editing Options > Maximum Characters Per Post

If you want to change minimum same thing just look above Maximum Characters Per Post.
 
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