What is the minimum size conduit/mast required for a 100 amp single phase 240 volt overhead electrical service?
Although you're probably safe by using the NEC as a minimum, this is going to vary somewhat based on jurisdiction and power company. You should contact your local inspection authority and your power company.
Service mast size will be specified by the local utility, though 2" rigid metal conduit is common. National Electrical Code (NEC) only has this to say...
National Electrical Code 2011
Article 230 Services
Overhead Service-Drop Conductors
230.28 Service Masts as Supports. Where a service mast is used for the support of service-drop conductors, it shall be of adequate strength or be supported by braces or guys to withstand safely the strain imposed by the service drop. Where raceway-type service masts are used, all raceway fittings shall be identified for use with service masts. Only power service-drop conductors shall be permitted to be attached to a service mast.
The local utility will not connect service conductors to a service point that does not meet their standards, so only they can tell you what they expect.