It really depends on the purpose of the document. A poster is usually meant to be viewed from far away, and I think that there are rules of thumb about the relation of the smallest font used to the overall size of the sheet. On the other hand, a technical drawing is meant for close scrutiny, so font sizes do not have to differ from normal office documents - it's more important to be able to cram in all the necessary information. And documents like architectural working drawing plans have lots of textual info like door tags that have to closely associated with a graphical object but must not cover it, so text styles for things like these tend to be rather small.
Anssi