The problem is how complicated Woot makes sizing when it really could be simple. Forget slim, thick, M, XL, etc. Sizing should be done like this:
Find a tee shirt you like and fits you.
Take the shirt off and lay it on a table (ladies you may not want to do this in public, but if you do we applaud your choice).
Measure the width between the arm pits (of the shirt).
Fold the shirt over at the highest part of the collar, then measure that folds length.
Find the shirt in this list that matches the width and height closest to your preference of bigger or smaller. Note some shrinkage (of the shirt) may occur with drying.
This gets rid of any doubt, yes, the collar or sleeves may be tight or loose, but we never measure that anyway. It sucks that we stick with sizing charts from the US military in 1940, when as a nation our dimensions have clearly "changed"...