I wouldn't go more than 50-75% of your filament diameter for max nozzle size, however that's just a guess. You need some back pressure I would think.
As for what to take into consideration, you will need a bit extra heat since you are pushing more filament.
As far as differences, here has been my experience.
.5mm low back pressure (High speed), and very hard to clog.
.4mm medium back pressure, rarely clogs.
.35 high back pressure and very easy to clog.
Once you cross the .4mm head jams become much more common. Some of this is due to being such a small orifice, but also most extruders can't push past small particles that jam. If you get an extruder/hobb that can REALLY push, they jam a lot less, unfortunately on 1.75, I only know of two... Ultibots 1.75mm and Griffin 1.75mm hobbs*.
I'm part owner of Griffin, but not affiliated with Ultibots.
As for what to take into consideration, you will need a bit extra heat since you are pushing more filament.
As far as differences, here has been my experience.
.5mm low back pressure (High speed), and very hard to clog.
.4mm medium back pressure, rarely clogs.
.35 high back pressure and very easy to clog.
Once you cross the .4mm head jams become much more common. Some of this is due to being such a small orifice, but also most extruders can't push past small particles that jam. If you get an extruder/hobb that can REALLY push, they jam a lot less, unfortunately on 1.75, I only know of two... Ultibots 1.75mm and Griffin 1.75mm hobbs*.
I'm part owner of Griffin, but not affiliated with Ultibots.