Well I would say that 2 lines of 16 characters each would be about the minimum. But ideally I'd like to pack as many lines and characters as could fit in, say, the space required for a 4" HMI.
Silicon Craft has several options. Easy to set baud rate and other info for the protocol. ASCII, RS232/485, Modbus are all options. I have used many of the 4 digit 7 segment LED for a machine that I upgraded. (The original used a 2 digit 7 segment LED that flashed a 4 digit code. Now there is no flashing just the code.) Would have used the 4 or 2 line LCD's but I tried to keep it as close to same as the original for the operators. A