Issue Description
There is not a particular size of file which is ideal, however, keeping your files as small as possible is recommended. As of 2023 we typically see PLN file sizes range between 100MB to 1 GB. If you PLN file is around 1 GB or more, we strongly suggest you check your file for any areas you can optimize.
Solution
There are some key areas you can clean up to shrink a file's size.
Embedded Library Objects
To check the Embedded Library just go to File > Libraries and Objects > Library Manager or just File > Library Manager, and highlight individual objects in the Embedded library. You will see a Size down below. Poorly scripted objects can be bloated. If you see any parts more than a few MB in size then you should consider replacing those objects with a better scripted one. Most objects will be in the range of a couple hundred Kb or less.
Delete Layouts
Make a copy of the PLN file for testing. In the test copy if you delete all layouts except for a single empty layout, and then re-save the PLN file, how big is the PLN afterward? If the file size decreases significantly, it is not a surprise. The Layout Book typically have more instance-based elements such as notes and 2D elements. This causes ArchiCAD to store a lot of data in the file. If it is the case and it is causing performance issues then we do have ways to split the Layout Book into its own file and the source model into another. It has its own trade-offs though. Help Center - Layout Workflow for Larger Projects
2D Elements
One last thing to check is for a large number of 2D elements like lines, arcs, polylines, splines etc in the project. Although it seems counter-intuitive, 2D elements cause more file-bloat than many 3D objects. It is because tools like the Line tool, Polyline tool, Arc/Circle tool are instance-based, meaning that they are primarily defined as they are drawn in the space they exist in. A GDL object or construction element, by contrast, is scripted and re-uses a lot of parametric scripts to keep the elements' impact on the file smaller.
Here are helpful articles we have related to file size:
How to Optimize Your Project Performance