As much detail as you can but if you have a relationship to with the venue, then chat to them. A stage plan is a model, it's not a substitue for a converation. With stage-plan.com, all instruments and items (including DI boxes, amps, plug sockets etc) are associated with a musician, which shows the venue who needs what. This gives the engineer/venue an idea of what you're expecting. Also don't forget to include any props! If the engineer knows that you've got a great statue of Stonehenge then he knows he probably can't put any microphone stands there!
A stage plot which shows a drummer, a bassist and a guitarist is great, but that's nowhere near enough – How many parts make the drum (how many kick drums, snares, number of toms, what cymbals etc), does the bassist require a DI or would he/she rather live microphones (or both)? You need to explain this via your plot. You may also want to read about whether you need an input list.
Provide the sound engineer with as much information as you can so he/she can use your stage plan and understand exactly what you need - just remember, the plot is not a subsitiute for a conversation. It's always great to chat! A stage plan is a plan, not a rule! Treat it as a guide! All models are wrong at the end of the day!