How to size overcurrent protection?
I would like to protect an OpAmp power supply from overcurrent.

Basically, you can't, because the current drawn by the opamp depends on the load on its output.
It's a high voltage opamp, +/-30V, so if you use a 2kOhm load, and the output is near +30V it will output 15mA into the load. If the output is shorted to ground, it will output about 25mA. Since you have 4 opamps, it's impossible to know if several opamps are providing legit current to loads, or if one is shorted, the supply currents would not be any different.
If the load will be a very high resistance, then the simplest protection option is to put a resistor in the output. You can wrap the feedback around it since the opamp supports differential input voltage equal to the supply voltage. Note this is important, if the opamp datasheet says &quot;maximum differential input voltage 1V&quot; or something like that, that would be exceeded if the output was shorted, and the output is connected to negative input, and the positive input is driven by another circuit. So this needs more resistors to limit current in the inputs.
Another solution would be to use an opamp with proper protection. Check TI search engine, there are several opamps with +/-30V supplies and proper protection, also cheaper than OPA445.