The minus sign means nothing. Same whether it has a minus sign or no minus sigh. The lower the number the smaller the font. The higher the bigger the font.

The Windowblind font sizes are actually controlled by the skin maker using skinstudio (SKS). 

Example: Open SKS. Load any skin except for a Stardock skin. Double click "Change skin fonts" Highlight the fonts and on the right side you will see what parts of the skin it's associated with. So if you were to change that font in SKS it would change all the parts associated with it on the skin. You will see if you opens another persons many of those associated places may be different. 

Since you are using Window 7 try this. You can always change it back to what it was. Open SKS and the WB you want to change or just try and see what changes. Open "Edit startmenu and Taskbar/ Edit startmenu. You will see "item mouseover most likely in two locations. At the bottom click on "Text". You will see text color and text font. Use the drop down menu on text font. Pick a totally different font from the list and click apply. Now open your startmenu and you will see the change. If you modify a WB and want to always have the WB you can go here in SKS. On the top left click on Make WB package. This will make a new WB with the changes. Now you don't have to do that because if you make the changes in SKS the WB on your desktop will remain with your changes. However if you were to delete if fome Windowblinds and load the orginial it would not include the changes. but if you make a new one and load that one it would onclude your changes.

This is most likely more information then you asked for but wanted to give you just a very small bit of other information.

Yes, negative number fonts are slightly larger than positive numbers for some odd reason. For instance, a -12 number would be a 10 on the positive side, and they would both look the same or extremely close. Why is this? I have no idea.