How to reduce the file size of .brd and/or remove unused embedded symbols

One thing you can do...

Unused symbols are automatically purged from the board. You can purge padstacks using the previous reply. Unless you have hundreds of unused padstacks, the board size will not be substantially reduced.

There is not much you can do to prune the design size unless you tend to load Gerber films for visual checking. If you use that model then you can delete these layers to reclaim space. 

I always check the drawing extents of the design to see if it is way larger than it needs to be and reduce it accordingly.  I have seen the database file size get reduced by simple pulling the extents in to what you need.   For example, having data that takes up 10" x 10" with an extents of 100" x 100" could probably be reduced to a more manageable size for a file size savings.

Of course make sure everything is displayed in your design to come up with the appropriate reduced size and I always make it slightly bigger for a little buffer.   I normally reduce it a little at a time with Database checks along the way.  It is very important to not reduce the database size too much and cut off design elements, it won't let you but it could lead to database instability.

To adjust the extents use Setup > Design Parameters under the Design tab in the "Extents" section.   NOTE: Always backup your design before doing any of these types of database changes and afterward make sure to run a full database check (Tools > Database Check) to verify that there is no issues after the extents reduction.  It is always good to check the Design Status as well (Setup > Status) to make sure everything is still intact as well. (I have never had an issue but it doesn't hurt to do a before and after for sanity sake)

Good luck,
Mike Catrambone

 Thanks