Zero thickness for Fluent 2d, and on z=0 plane. There are additional rules for axi-symmetric models.
I suspect you've got some conflicting edge sizing set - look at what's set, and what impact that'll have on the local area. It's more common with Ansys Meshing to use size functions with fewer local controls rather than the "old school" approach of setting everything manually. Have a look at the courses on Learning and Tutorials in Help.
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