The battery size is not important. The draw of everything pulling FROM them is what matters.
I personally am a fan of Blue Sea marine battery switches like
https://www.amazon.com/Blue-Sea-Systems-Battery-Switch/dp/B00445KFZ2 (although that one is pretty huge) or some of the generics that look like
https://www.amazon.com/Ampper-Battery-Disconnect-Isolator-Vehicle/dp/B07413JWLD (I use three of those in my current 5er - one on the batteries, one on the solar PV inputs so I can disconnect those if I need to service my solar array, and one from my batteries to my inverter, so I can service my inverter without disconnecting my batteries completely).
I suggest going with a switch that can handle at least 3x the current that your fuse panel is set up for, so if you have say a 40-50A main fuse or breaker I'd start at 150-200A. If you have an inverter or other high-draw load, look for 275A+. You aren't going to draw anywhere near that - a typical house load might be 10A if your furnace and a few other things are all going, a typical charger might put out 30A, and a high-end "fast" charger might be 100A. But the act of turning these switches off can cause "arcing" at their terminals. They're designed for this so it's nothing to panic over, but it does shorten their lifetimes and the beefier switches will just last longer. They're so cheap, why not go a little above what you really "need"?
You should disconnect BOTH cables before cutting your positive. Disconnect your negative first. This used to be a safety thing - it greatly reduces the chance of getting arcing/sparks when you disconnect your positive cable, and lead-acids often have a risk of having hydrogen gas out their vents. Modern batteries are usually sealed / AGMs and this risk is much lower (though never zero) but it's a good habit to get into anyway.
I STRONGLY recommend that if you're going to cut your cable and add terminals to it that you get a hydraulic crimper. They're cheap on Amazon and even the cheapest will do a far better job than pliers or any other hand tools. Battery terminals are notorious for corroding over time and you're going to want a "gas tight" connection. That's created by deforming the copper in both the lug and cable. Never solder these things. Solder can wick along the individual strands in the cable and create fatigue/stress points under the insulation where you can't see it. Over time as you travel, individual strands can break with no way to know this has happened, creating high-resistance points in the cable that can eventually lead to (at best) electrical issues and (at worst) fires. You won't find solder anywhere in your car's connectors and it's more or less forbidden in aircraft.
Since your cables already have terminals on them, the easiest option is usually just to use what you already have. You can get pre-made 6" - 3' battery cables with their terminals already attached. If you're planning this cut because your cable has a top-post clamp-style terminal instead of a ring terminal, be aware that you can also buy "top post terminal adapters" as well. Saves a step. Just disconnect your negative, disconnect your positive, positive -> switch, shortie/jumper from switch to battery, reconnect negative, done. No cutting or crimping.
If you do install your own terminals, consider also buying "adhesive lined" heat shrink. It's worth the extra 37c to get a water-tight connection.
