Do NOT just choose your regular lab coat size. These will fit differently. Measure!
Using a tape measure or a string and ruler, measure the circumference of your body without pulling it tight. Wear your normal lab clothes and include those in your measurement.
Below are the maximum measurements each lab coat size will accommodate. Make sure all 3 of your measurements are at or below these numbers. For a loose fit, go up one more size. The "Waist" numbers are before using the belt, which pulls in up to 8 more inches.
(centimeters) / 2.54 = inches