Written by MIke Cohn talks a lot about this in his book Agile Estimation and Planning. You can use any measure to size stories.
Teams use different sizing techniques:
T-Shirt Sizing
X Small, Small, Medium, Large , Xtra Large
or
Coffee Sizing
Small, Tall, Grande
You can pick any sizing technique. Make up one if needed.
This is mainly done as we as humans and as managers are better at abstract terms. If we use hours as a way to size stories,then the managers in the room have questions, teams dont immediately feel comfortable with hours.
Sizing keeps the planning meeting at a fast pace.
A general measure you can use for T-Shirt sizing is how many days it take to complete a story
Small story – 1-3 days
Medium – 3- 5 days
Large – 5 – 7 days
X Large > 10 days
In order to convert a story size to a number, you can use either factorial, Fibonacci, or Squares.
If in a sprint your team does two small stories and two mediums then your velocity is 9 * 2 + 4 * 2 = 26 ( if you are following the squares technique). Square is simple and works quite well.
Agile Practitioners Guide 
In reality there is no corelation between days and Size of story. In my mind i do corelate X Small to something that we can do in a day to two. This is code , test document everything as per defn of done