For a standard text message, 153 characters will use one credit. The standard font is Ariel, size 13. You'll be able to see if you're using the standard font when you're writing your SMS as you'll see how many characters you've used and how many per credit you have.
Is the SMS signature included in credits used?
Yes, the SMS signature is included in the number of credits used. You can see how many characters your signature has when you're writing your SMS.