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 (the layout and How Do I Make An Input Or Monitor 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!