What type and size conduit should I use for my service entrance cable?
How deep do you go? You can give metal Rigid (/RMC) or IMC conduit only 6" of cover.    Other conduit needs 18" of cover.   If you want to direct bury, you need 24" of cover. 

Since you are downstream of a main panel that presumably serves other loads, this is not service wire, this is feeder, and needs to be sized as feeder not service conductors.  You need to run separate neutral and ground wires, unless you are running in IMC or Rigid conduit, in which case you are allowed to use the conduit shell as the ground wire. 

Don't run cable in conduit, that is the mark of the amateur.  It is also a very, very hard job.   Either direct bury at 24" or use THWN-2 wires instead of cable.