I am just shy of 6 foot.  According to the chart one might suspect I would ride a large 59cm frame, however God gave me a 32 inch inseam and accordingly ride a medium 56cm frame.  I could ride a large, but I would stand on my toes at a stop light rather than flat footed, which makes starting and stopping easier. 

What size cog do I need?

Cogs, also known as sprockets come in varying sizes.  Road bikes may come with up to 10 cogs on the rear wheel cassette.  When you make the decision to go single speed, the challenge is which cog is the right one for you? The easiest way to determine the right cog is to ride a bike you already own or try one at a bike shop. Most people find a specific combination of front chain ring and rear cog to be the "sweet spot".  If you know the brand and model, you can look up these settings on the Internet.  For instance if there are three chain rings up front , the specifications on the manufactures site will tell you the  three sizes (i.e. 50/39/30).  See which one you are using.  Then look at the rear cassette and see which cog you are in and check the range on the site (i.e.11-32). The next step is to determine the "gear inches" you are riding.  The best place is provided by the godfather of bike mechanics, Sheldon Brown.  Sheldon died in the spring of 2008, but his advise will live forever on the Internet.  Go to Sheldon's gear calculator. Make sure you are calculating for gear inches.  If you are having trouble calculating your sweet spot just drop me an email.    Coaster cogs come in a few sizes from 14T (teeth) to 22T.  By default I will install a 18T cog on your bike. When combined with the 48T chain ring you will get 72 gear inches, a bit stouter than the 67 inch Surly Steamroller. The 22T is 59 gear inches, which is slightly bigger than a beach cruiser at 52 gear inches. If needed, I can put a 44T Sugino Zen Messenger chainring on the crank ($50 additional) to give you more choices.