I have had the same question when I was setting up the calcium reactor, and I can tell you from the experience, oversized CA reactor is not a problem I am running a GEO 618 on a 70 gallon( it is rated for 300 i think) Undersizing it could be problematic, as my LFS is running 2 reactors on their Large SPS system because one just couldn't keep up.
The only difference is change rate of media. If you get an undersized reactor you need to swap out the media more frequently. Oversized the media takes longer to break down as you arent dissolving as high of a % of it.
Would there be a problem with it taking longer to break down? Does it somehow become less effective? Does the media need to be kept at a certain level for the reactor to function efficiently or can you pretty much let it run down before adding new media?
No problem with longer breakdown times and doesn't become less effective. My current schedule is to swap out the media in about 8 months. Though that depends on my consumption rate as well as I add more sticks. I would swap it all personally and take that time to clean the reactor and pump.