The user name font can be changed but it's not directly editable in the settings. I know for even slightly longer names, it gets cut off.
The font is stored in the skin and the skins are all located in C:\Program Files\Classic Shell\Skins. Go there, and copy the skin whose font you want to edit to some location like the Desktop, rename it and then open the .skin file in the freeware Resource Hacker (http://www.angusj.com/resourcehacker/). You will need basic knowledge of Resource Hacker otherwise you can tell me the skin, font and font size you prefer and I will edit it for you.
Once you open the skin in Resource Hacker you can expand the first resource called Skin\1\. From Resource Hacker's View menu, click Find Text and look for the parameter called User_font=. Change that carefully to another font, font size, normal, bold or italics and click "Compile" above. Then click Save from Resource Hacker's File menu. Copy the modified file back to C:\Program Files\Classic Shell\Skins. Then open Classic Start Menu settings and switch to the Skins tab. Select the modified skin from the list and click OK.