Technically a 256GB drive is big enough to support two bootable partitions (although I would much prefer two APFS volumes for numerous reasons). After all MacBook Air’s commonly ship with only a 128GB SSD. Whether it will be big enough to support three boot systems and at least four volumes (Recovery Drive, Mojave, Catalina Boot, Catalina Data) is another matter. It will depend on how many applications you have in the Mojave and Catalina Data volumes, how much data you have, and how much use you make of a common iCloud Drive for your Desktop and Documents folders.
Personally, by making maximum use of iCloud Drive storage including streaming all my music, movies, audio books, books, etc rather than keeping them on the SSD, and only having optimized photos on the SSD I am using just under 200 GB on a 512 GB SSD.
With a 256GB SSD you may want/need to consider an external drive for your Mojave volume.
