Personally I think you're going to deep (for an interview) if you start drawing class diagrams, unless they ask for it.
Last time I had this question I ended up drawing the different layers (3 tier app), explained how the assemblies were mapped (cause this was something 'odd' for the project in my opinion), in which direction the layers had dependencies and the direction of dataflow.
You can go deeper in specific components if you feel you have to. But I never went much deeper than "we used Workflow Foundation for this part of the business logic, because of that reason". This gave me the chance to point out we used a certain technology used, without wasting time on trivial things like simple POCO objects.
More important is showing that you understand the architecture and why it's done like that. Even better is to point out points of improvement (if possible) and explain why so. If on the other hand you think the design is 'perfect', you can point out a particular part of the design that doesn't seem logic for an outstander and explain why it fits for this project.