This explains how to quickly and easily create a multipart (multivolume) RAR file using Gnome / File Roller in Ubuntu.
First, locate the file(s) / folder(s) you want to compress and highlight them. Next, right-click and select Compress...
When the Compress dialog appears, enter a filename for your archive and select rar as the extension. Click the arrow next to Other Options to expand the dialog. Check the box next to Split into volumes of and enter the desired max individual file size here.
Click Create and the compression process will begin. Note that on extremely large multipart archives with lots of part files, a heavy amount of disc activity may occur, and the archiving process (and some other programs) may stall or momentarily freeze because of the disc activity. Be patient and allow the process to complete.