How can I change the tag size in the tag manager?

My tag manager size is preset to 0 and no elements are listed below it. When I try and change the 0 to any other number and hit enter it refers back to 0. This means I can’t add any new tag to any object. The same thing happens with the layers section. However, I am able to change the size of the sorting layers size to anything I want and it adds new defaults. I am using unity 5.

I have no idea what are you talking about, but here is how to add tag/sorting layer (marked with red):
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P.S. Recently I’ve answered a similar question that shown the usage of Debug mode with no tags, I guess that is your issue. I’ll just add link HERE.