Soft Starter Sizing for No Load Testing of Motors
It sounds like you want to find the smallest soft starter you need to just do basic unloaded spin-up testing of repaired motors in your shop. In that case all you will care for is voltage requirements and overload current ratings of the starter.

AC motors will draw their locked-rotor (typically 6x for NEMA B motors) current when starting up under load. Even unloaded, there will be a brief (several seconds) of higher-than-nameplate current. This will not really be visible for small (&lt;25-50HP) motors, but for larger motors with large rotors you can see 2x or maybe even 3x nameplate as the big, heavy rotor slowly spins up.

Having said that, however, any starter worth it's salt will have a decent (3x to 6x) overload rating, and should also feature a current ramp start. I'd size the starter so that its overload rating is twice your biggest motor's nameplate (e.g. a starter with an overload current rating of 400A should be fine for a 200A or even 300A motor) -- just make sure the current limit is set up and it'll keep the current within the safe operating area of the starter, and it's up to you to make sure that the motor can start up unloaded with the current limit in place. The motor will be a slow start, but your starter and wiring should be safe, which is the whole point. The starter should also have short-circuit protection to protect everything from wiring mishaps or a badly rewound motor.