Entropy of String States at fixed Mass and Size

We provide formulas for the entropy of free-string states depending on their mass, charges and size, both in bosonic and superstring theory (IIA or IIB). We properly define these quantities in full-fledged string theory. We then investigate the corrections to the entropy due to self-interactions of the string for states with fixed mass, charge and size, both for BPS and non-BPS configurations. Again, the analysis is performed using string theory techniques.