Recently I try to import repository with dump file. The file size was something about 4GB and it doesn't want to work, file was rejected. I think file was ok so I have a question that I can't find answers in documentation.
If I import dump file to SCM through UI, what is its maximum size?
I don't think that scm-manager limits the size. I think the upload is limited by your browser, most of the common browsers have an upload limit of 4GB.
If you have access to the machine, you could create a repository with scm-manager, copy the dump on the machine and load the dump into the created repository with the svnadmin tool.