Thanks for the replies - I didn't get a chance to check all your advices until today, as my "darkroom" is available on weekends only, and even that only when there's not much light outside
Anyway, I do have a lens panel which brings the lens closer to the negative (Siriotub), and lens indeed has to be closer to the negative the larger the image gets. It turns out that I am just barely able to enlarge to some A4 paper size, which is about the declared value for the lens-condenser combination that I have.
Right now, I can remove the cloth thing on the lens holder, which will give me some extra 1-2cm of space. But I didn't expect Durst meant to work this way?