We all fluctuate between sizes, it's natural. However, if your shirt size is medium or large, and there is only a small left of the speckled chambray you've been eyeing, don't pull the trigger. Unless you plan on losing 25 pounds in the next six months (and shortening your arms by three inches), this is a poor purchase. The item will just collect dust in the back of your closet or make you look foolish, both bad outcomes. It's never cool to have a shirt you can only wear with the sleeves rolled up or top button undone. Instead, you can combine the money you saved on the four items you don't wear (because they don't fit), and buy something that's your size which you'll wear all the time.