No problem at all.
You can even stick them to the inner chain ring (or the bolts/arms if there are any on your rig) using epoxy or even just magnetically. The grade 52 magnets are quite fierce and will hold in place with great tenacity if your harvested magnets don't stay in place (see; https://www.kjmagnetics.com/products.asp?cat=1
The effective diameter of placement doesn't matter, but get the polarity right. Mark them with a Sharpie or similar before punching them out. If your sensor has a LED indicator, then when you're done with your new wheel, wave the sensor by each magnet in the direction of crank rotation and verify that each magnet is installed correctly.
The big thing here is the number of magnets since that determines the angle of rotation before the electronics gets some notion of what is going on - either starting or stopping. More magnets = better response. In the good old days, some wheels only had only 5 magnets, now you can get sensors with 24 poles (!). That said, even 5 magnet wheels work - only the start and stop are affected. The only caveat here is if this sensor is part of some 'smart' controller or unit that expects a specific number of poles then changing the number of magnets might slightly change getaway behavior, etc since the magnet-reported cadence will not be the same with the original number of magnets. This is not generally an issue. In any case - easy to try and verify...