Chest. Everything else—length, sleeve, shoulder—falls into place if the chest fits. Grab a soft tape measure and wrap it around the fullest part of your chest, keeping it level under your arms. Don't puff up or hold your breath. That number is your starting point.

Compare it to the Windbar size chart. If your chest lands at the top of a size range, you're in slim fit territory. If it's at the bottom, you might find the tee a bit looser than expected.