Hi Dmitry,
Mike Yearwood advocated this one class per classlib and with common classes that is what I do. We have over 300 classes in a common folder. Most project specific classes are in a single classlibrary, always named the same, MyApp. Sometimes for ease of use we divide things into a few more.
I have EXEs that are 42 MBs, mostly because of all the template, XLTX, type files that are included. We don't have any issues with performance, so I don't really worry about the size.
When we first started with OOP developement we put common controls, forms, etc, into classlibrary files, grouping by type. We are still paying for that. If I reference one of the old classes, suddenly every one of those original classlibraries is brought into my project.