There are several fairly standard size nozzles that range from 0.020" to 0.060" or so inches in diameter.
If you want your cut-out to work with just about anybody's system, then keep your smallest inside radius above 0.030", and you should be fine. That's overkill for the average shop, though.
If for artistic reasons you want to go with the smallest radius possible, then you can find a shop that has smaller nozzles available. Its reasonably common for nozzles to run at 0.021" in diameter, for example, so you can do a radius as small as 0.01" or so.
Tip: If your goal is the sharpest inside corner possible, then draw it 100% sharp, with no radius at all. Then the corner radius will exactly equal the jet radius, and you will avoid tool-offset issues that might otherwise occur.
I should also mention that there are smaller radius nozzles available, but they are not commonly found in your typical job shop.
Hope this helps.