Split a file into 19<B chunks?

I want to split an 81MB file into 19MB chunks. I run the Opus CLI, get the "Directory Opus CLI v1.1" prompt, type >SPLIT, get the "Split File - Directory Opus" dialog. Chunk size there is either "automatic" or there's a selection of common values. How do I enter 19MB so they split into 19MB files?

Just enter 19 mb.