the limit of the suction line is the refrigerant velocity that should be at around 7m/s in vapor rising pipes to ensure that the oil comes back to the compressor. if you have a continously falling pipe and no rising pipe diameter doesn't matter.
you can go under this 7m/s if you are sure that the oil flows back to the compressor. I've seen on an experimental unit a velocity of 2,8m/s that was enough to get the oil back through a rising vapor suction pipe.
You should also care about that above when you are using an oil seperator because they have an efficiency below 100%.