I have been converting for some years now (5?) Epub to 9×12 PDF using the Prince PDF plugin published here by Jellby. (a more recent Sigil version of it made by Doitsu exists).
djvu seems quite an interesting format and I am looking for a similar capability as it seems that it could -potentially and ideally- produce even smaller files (at the same quality level).
Up to now, I found that the online site
convertio produces directly nice documents in djvu format from epub but it does not seem to offer any choice for dimensions. (it produces standard A4 files). They offer a "conversion API" choice but I failed to spot any dimension choice there -maybe I missed something.
Other sites pretending to convert Epub to djvu do much worse. Sobolsoft uses a two step process, converting first to a temporary PDF and them converting to djvu.
I could already do the same, for example using Jellby's plugin and then the excellent opensource
pdf2djvu software. But the output djvu file exceeds the PDF size which defeats my initial purpose...
The
STDU viewer is said to be able to view (among many other formats) Epub files but I failed to do so with every single Epub I used.