I'm wondering what is a usually tip used to decorated sugar cookies? Not for flooding as that can be done with a ziplock bag as well but to get a nice clean thin but not super thin outline what # should I purchase? Thanks

I think most use #3 for what you describe.

outline and fill with the PME #3, details with the #1.5.   If I had to pick only two tips for cookies for both corn syrup glaze & royal icing, these two tips would be my choice.

http://www.globalsugarart.com/pme-decorating-tips-c-641_737.html

This person uses tip #2, so would 3 give thicker lines?

http://www.mycookieclinic.com/2013/03/sugar-bunnies.html

Yes.. #2 is smaller (thinner) than #3. They are only about a dollar each, and less if you have a coupon (Hobby Lobby/ Michaels). I would buy a few and try it to see what look you like.