All the measurements below are for the garment itself. To select the right size, take a similar item, place it flat and measure it as done below. 

INDOOR DELTA T-SHIRT

Indoor Delta Polartec Tshirt

HEIGHT & WEIGHT CHART

Here's a benchmark guide to further help you with sizing according to your height and weight. This is based on past purchases and actual product fit. The range varies depending on your body shape (hourglass vs pear, vs rectangle), this is why there may be sizing overlap, which means you could fit both. Your choice will then boil down to how you like your fit, if more "snug" or more "comfortable".