Is 22MB too big for dll file ?

It depends on how you use the components in the dll and how they are constructed.  If you use all or most of the components in each application that invokes the dll, then you might as well keep one big dll.  If you use the dll in several apps and each app only uses a subset of the components, then it would make sense to split it up.  Another reason to split it up would be so you can develop the independent parts withoug rebuilding the whole thing.  If you have a couple of common base clases upon which everything else is built, it might make sense to put those in a separate dll as well.

Without more details, it's not a simple yes/no anwser.

My application has 10 product catalogs and this particular 22MB dll file contains components for one of the catalogs.
It is used only when user is using this specific catalog otherwise it is not used.

I have used DLL's with over 100MB in size (many resources inside it) without noticing any problems. The needed parts are paged in and out just as needed, so that there isn't a long load time nor does it block tham much physical memory. No problem.

22 MB is very less, the momory size has increased a lot,
But you have to check the client RAMs
and if it is resources i.e. images
They come and go if you are not loosing references in your application.. GC takes care of them
I mean, if you are not opening many images in one time in the application

Thanks