Alain here
If you're talking about technical limits, the maximum length of a Word document is 32 MB excluding images.
If you are talking about self-imposed limits, I guess you need to pick your own challenge, between sticking to the original length (but compromise quality sometimes) like in most commercial projects, or ignore it to provide your ideal translation.
Both are legitimate choices, since you can't transfer your translations in the game anyway.
I will leave it to UBE to answer about the editable files, but PDFs can be edited with OpenOffice if needed.