When shopping - bigger can be better, but if going really extreme, can be harder to regulate. Look for one that allows you to control the amount generated in smaller steps. My Circupool RJ45+ does it in 5% increments. Some of their other models, and other brands, do it in 10% or 20% increments, which could mean you fiddle with it every few days or a week going from too little to too much. It can only be used when the pump is running, so that factors in to the "how much" balancing act. Running the pump for half a day with it at 100% is the same as setting the SWCG to be at 50% strength for a whole day. So if you do end up with one with bigger steps, the amount of time running the pump can help fine tune the end result.
Mine is about 2x the recommended capacity, and I do run it at about 40-50% 24/7. Works out fine, and in theory it should last 2 times longer than a smaller one. I do kind of wish I had gone 3x bigger, but at the time their "deal" was different...and except for even longer life, there is really no other reason to desire it. There is a point where it would be lasting longer than I do!
What's bad about having one? Not much. I can taste the salt, no one else has noticed when asked. Day after day I look at the crystal clear pool, with nothing to do. And spend very little on pool chemical stuff.