In regards to large projects we will break it down into chunks that can be developed. For example, we are creating a piece of software that allows a user to design a trip and view trips they've created. So we broke it down into a set of stories:
1. Create a trip wizard
2. View current trips
3. Edit a trip
4. View reporting for trips
And even those can be broken down further I.e page 1 of wizard, 2 of wizard etc, that way the devs can work on part of it.
For smaller projects e.g adding a new piece of functionality into the create trip wizard, we can tweak the process to skip sketches and instead amend the prototype directly, but this would only be for individual input fields rather than a whole new page. We will still review requirements and review the changes with stakeholders before dev starts. We also usability test small changes as best as possible.
We try to stay 2 sprints ahead of devs so they have time to plan their stories before doing the work.