For maintaining good detail in a good negative at a close viewing distance could go to 8-10x. I've done images where those technical quality measures are pretty useless, I have a 35mm Tri-x neg of a horse that i enlarged to around 20x30. Printed it as lith and it worked out well, but it is a lot harder, I'm sticking to medium format and up for nor s that I don't need to deal with film flatness and tiny negatives, My current favorite seems to be a Hassy, I sometimes use LF, but the hassy gets all the good shoots. I'm actually thinking about dumping my Tachihara 90 and 150mm wit a bunch of holders.
Bottom line - MF will suit most of your work unless you shoot a lot of architectural shots then it is the movements and not the film size.