My recommendation from my diving experience, and from working on a boat, is get a 6 footer. I personaly have one that is 6 foot tall, and is a closed bottom so oral, or LP line inflate. It has reflective strips on top and a pocket you can slide a strobe into. Once you have it, practice as people have said, be able to deploy that, and get it well filled ( make sure it has relief valve). The other thing is keep tension on it. When I accend I stay slightly negative and use the line as my guide, keeping tension on it so it is upright. So many people deploy it and it hits the surface nad flops over rendering it ineffective. Keep tension, and when you get to the surface hold it upright until your boat is close enough to protect you.