burning one hour per inch is usually the best way to go, although if it is a 4in candle and is about to spill over after 2 hours, go ahead and blow it out. The wick is too big. this creates a "memory" melt pool for the wax and it is less likely to tunnel down the middle. also, once you blow it out, gently round the edges in with your hands. this is called "candle hugging." It helps keep the sides strong (you've probably seen the little slits that can all of a sudden become big slits and spill everywhere).
Just b/c you do this does not guarantee a good burn. I've seen candles from Pier 1 and other places with little bitty wicks that look like floss for a 3 or 4inch diameter candle. There is no way that candle was designed to burn right.
Most people who make their own candles or who started out making their own and then grew bigger will try very hard to make sure the right wick is in the right size candle. A LOT of factors go into picking the right wick for each candle size, fragrance and type of wax.
My general rules for picking a good candle: 1 - Make sure the wick is as close to center as possible. You can't really change that once the candle is made, so don't think you can fit is as it burns. 2 - The wick thickness should make sense - like above, if it's a piece of floss for anything larger than 2in, run far away.
3. If you are buying from some who makes their own (like me), go ahead and ask what kind of wax (parrafin, soy, etc) is used and what sort of testing is done. Just don't ask how you can do it yourself. I've spent years making and perfecting mine, so don't think i'm going to tell you all my secrets.

If a chandler cannot tell you these basic things, they probably don't know what they are doing and don't care much about the quality of the candle.
and always remember to never leave a burning candle unattended (including while someone is sleeping) or with in 8-12 inches of something flammable (and that is a minimum).
happy burning!