The upside?
A larger diameter piston will displace a greater amount of fluid for a given amount of pedal travel. This will be good if the pedal currently travels a bit far, even when the brake shoes are fully & correctly adjusted.

The downside?
You will generate less line pressure for a given amount of pedal pressure, so in order to restore the previous level of line pressure, you will need to press harder on the pedal.

If the resulting pedal pressure is still within acceptable limits, then it's all good,but if, after a while, your right leg looks like Arnie Shwartzeneger's & your left looks like a normal persons, then you just gotta know that you need a booster with the new cylinder.