Count what is on the drawing, then add
Start with the I/O list from the electrical drawing and count inputs and outputs separately — they are rarely symmetric, and a controller chosen on the total can be short on one side. Then add 20 %: a typical machine gains a safety sensor, a second temperature probe or an extra solenoid between the prototype and the third revision.
On the FAs series the frames run 16, 20, 22, 32, 40, 50 and 66 points. The jump from 32 to 40 is small; the jump from 40 to 66 is where a machine outgrows its controller entirely. Choosing one frame above the count is usually the cheapest decision on the drawing.
Expansion or a bigger main unit
FAs and FAT accept 64 S-series modules over CAN at 1 Mbps, which is more expansion than most machines will ever need. FGs and FGm accept 8, FGR uses its own A-E/FGR-E modules, and PRO/MAX chain five PRO-E slices from a START head.
Expansion is the right answer when the extra I/O is physically distributed — a remote sensor box at the far end of the machine. It is the wrong answer when the extra I/O is in the same cabinet, because each module costs more than the difference between two main-unit frames and adds a CAN address to commission.
Relay or transistor
Relay outputs suit mixed AC and DC loads and survive an inductive kick better than a transistor without a flyback diode. Transistor outputs are faster, last longer under frequent switching and are the only ones that carry pulse trains.
The rule that survives contact with reality: transistor for anything that pulses or switches more than a few times a minute, relay for contactors, solenoids and anything the customer might wire to a different voltage than expected.
Spare inputs are cheaper than spare controllers
A machine that ships with four spare inputs and two spare outputs costs a little more at the quotation and saves a controller replacement at the first field modification. It also makes the commissioning engineer's life easier: a temporary sensor can be wired without a trip to the wholesaler.
The same logic applies to the analogue side. A temperature module with four channels where two are used leaves room for the second zone the customer will ask for, and 16-bit resolution costs no more than 12-bit at the same channel count.