The shirts vary slightly depending on manufacturer and material but this guide is pretty accurate.  

The cotton shirts run true to size.  They will shrink some if put in the dryer.  The tall sizes are generous shirts through the shoulders, arms and body.  Depending on how you like to wear your shirt 6'3" and over would probably prefer a tall size.

The fitted heather poly blended shirts also run true to size but they are slim fitting in the arms and slightly through the body.  The length is a little longer than the cotton.  They will also shrink a little in the dryer.  If you prefer a loose fitting shirt consider a size up in these.