When purchasing a dress shirt for a man, the first thing to remember is purchase the best you can afford, these are the work horse of your wardrobe so make sure you buy quality or you will be replacing it sooner. Fit is important of course, you want the cuffs to come to just above the top of your thumb, all buttons need to line up and not gap, the collar at the neck needs to be comfortable without gaping. Look at the quality of the stiching, is it even and straight. Here is another trick of the trade, if you have long arms show more of the cuff below your jacket sleeve it helps create the illusion of shorter arms. If you have short arms have very little of the cuff exposed as it helps create an illusion of a longer arm.