Clicking on the map (shown here at top right, but on a small screen you may need to scroll down to find it) allows you to limit the search to a geographical region but will use records of all types.
The gold-standard for efficient and effective searching is to go to the bottom right and select Browse all published collections (even though you are not going to search not browse!) This opens a new screen that lists the names of all (currently 2,518) searchable collections. Fortunately, it also offers an easy water to filter that list to locate what your really want.
Postscript: If you want to review the process for searching all the records of a particular geographical area, take a look at this report on a previous meeting.