I use only Gildan brand ultra cotton tee shirts with a weight of 6.1 oz. Although they are prewashed by the manufacturer, I have found that they shrink about 1" in width and length with the first wash. The sizes are unisex.
I personally measured the shirts, and for the width I measured across the chest under the arms. For the length, I measured on the back from the top of the neck to the bottom of the shirt.