The stock steel fuel tank on the 96-Present DR650SE's has a capacity of 3.4 gallons unless you have a bike that was first sold in California. The capacity is 3.2 gallons for the CA bike due to part of the emission system being inside the tank. Both tanks have a reserve capacity of .8 gallons and a vacuum operated fuel petcock with "on, reserve and prime" positions. Prime is used for filling the float bowl of the carb after service work (or any other reason it may be empty) without the engine running to produce the needed vacuum to open the petcock.
Purchasing a larger, lighter plastic fuel tank is a popular mod for the DR650 to increase fuel capacity for longer riding sessions between fill-ups. As you will see below you have some choices!