Your PFD should fit properly, and according to international sea kayak authority Derek Hutchinson, it should give you between 16 and 17 pounds of buoyancy. When trying on a new PFD, test the fit by sitting and making sure it does not ride up to the level of your ears or higher. You want the PFD to fit snug enough so that the shoulders do not ride up and that the only thing that can fit between your shoulders and the PFD shoulder straps is your hand. You do not want it to be so tight that it restricts your breathing in any way, even when you are taking a big, deep breath. Your shoulders also, should have full range of motion should you need to swim. Choose PFDs with heavy-duty diving suit-type zippers and buckles and avoid ones with only elastic around the waist.
PFDs should have closed-cell foam inside the external shell material. Modern PFDs are usually made of a 500 denier rip stop material that will hold up through many years of wear, but if you are borrowing an old PFD or are considering purchasing a very used one, inspect the seams and construction to make sure there are no weak points or tears. Retro-reflective tape pieces will aid in night visibility. The inside fabric is usually softer so it feels less abrasive against the skin. The skeletal harness of webbing, which is structurally sewn to the outside of the PFD, is used as anchor points for towing or attaching accessories. Give these pieces of webbing a good look too when purchasing a PFD.
Many PFDs today are designed in a variety of colors but whatever color you choose it should be bright. As mentioned in Derek Hutchinson's "The Complete Book of Sea Kayaking," he suggests yellow, red, or orange are best. Gender specific fits are also available, and may have multiple little pockets to stow personal items. Many are also made with a sheath for a quick release knife on the front of the torso. Whether you choose a PFD with one pocket or five, pink or green; knife or not, it is important to be satisfied with its design so that you will be more inclined to put it on each time you kayak. This is especially important for kids, so they won't have any aversion to wearing them.