Typically, jurors in United States District Court are summoned from either the entire district, if its geographic footprint is small, or from a fixed subdivision of the district. State charges arising from a felony offense alleged to have been committed in downtown Portland would be tried in Portland, with 12 jurors drawn from Multnomah County.
Federal charges arising from a felony offense alleged to have been committed in Portland would be tried in Portland, with six jurors drawn from Clackamas, Clatsop, Columbia, Hood River, Jefferson, Multnomah, Polk, Tillamook, Wasco, Washington, and Yamhill counties.
The calculation about whether jury nullification will allow a defendant to walk is completely different when you have a smaller jury drawn from a much more diverse area.