If you have a library that has gotten too large and you want to split it into multiple, smaller libraries, there are a couple different ways you can go about doing so.
First, click the “+” button and select “New Library” to choose the location you want to create the second library.
Then, drag one or more albums from the album list of your big library onto your newly created library. This will copy those albums over to the new library, including all their dates, keywords, descriptions, etc. After you have copied the photos to your new library, you can use Photos to delete them from the big library, to clear up the disk space in that library. You can repeat this process with small or large batches of albums and any number of libraries to split up your collection into smaller, more manageable pieces.
One common way of splitting up a library is to split things up by date, e.g. so you have one library per year. The easiest way to do that is to create a smart album in your Photos library that uses the “Date” field to show just photos from a certain range of dates. For example, if you wanted to make a library with just photos from 2013, you’d set up a smart album with a date range from Jan 1, 2013 to Dec 31, 2013. Then in PowerPhotos, you can just create a new library then drag that smart album over to the new library to copy all those photos across. Then just repeat for each library you want to create with a different date range.
Duplicate & Prune
Use the “Duplicate Library” command in the File menu to make a copy of the library folder that you want to split. You can then open one copy of the library and move whatever photos and albums you don’t want in that library to its trash. Then open the second library and move the rest of the photos into that library’s trash. After double checking to make sure that no photos have been trashed from both libraries, empty the trash of each library. This method is useful if you want to avoid some of the copying limitations that are imposed on PowerPhotos by what Photos itself is capable of.