Bigger than needed can be a problem. Slow cookers are designed to work best when filled 1/2 - 2/3 full. A five quart is a good mid range size for occasional needs. Ovals are more user friendly in that you don't have to cut small roasts in half to fit them. A true slow cooker has the heat coming from the sides surrounding the contents. Those that bill themselves as all purpose with the heat only at the bottom are basically just simmering.

Oh well -- forget what I wrote. I know nothing.

No need to feel rebuked. Your initial answer started with "depends." That's still valid. Plus your Instapot has more flexibility than most slow-cookers and does a good job with lesser quantities. Slow cookers vary in the design and placement of heating elements which is a reason for a generic ½ to 2/3 recommendation for "best" performance. For some slowcookers, the design and placement of heating elements may be a problem with small quantities getting too hot or not hot enough. Other slow cookers will work fine with quantities of less than half full or more than 2/3 full. My two Crockpots do.

But, getting back to gsshal's question, "how big?" Depends on a lot of factors, but here are some rules of thumb.

Are you feeding small armies of ravenous children and ranch hands, do you regularly have large gatherings, or are you planning on making a month's worth of meals to portion and freeze? You might consider a Nesco countertop roaster which can be converted to a 30-quart slow-cooker and buffet server.

Hamilton Beach and several other brands seem to have units with 8.5 and
10 quart capacities. Are you cooking for large families of growing
children who consume mass
quantities of whatever you make and make allowance for leftovers? Do you often cook 10 pound pot-roasts or large briskets? For those things, you may want a slow cooker in this size,

Otherwise, the most readily available slow-cooker models are around 6 quarts capacity. Some are 5.5 quarts, some 7, but those size variations won't make noticeable differences to most cooks. Slow-cooker cookbook and on-line recipes generally assume a generic slowcooker in this size. This is what most folks wind up with.

If you are only cooking for one or two, want to make smaller quantities and avoid leftovers, and will never cook anything of any size (say, beef pot roasts or chicken quarters), then you might consider a 4 quart model. Go below that size and you are pretty much looking at party-dip heaters or maybe overnight oat cookers. (Heat control in some of the smaller units is not very good.)

Or, are you mainly interested in the foolproof fire-and-forget convenience of beans, grains and stews cooking while you are out working for the day?

For that, I second nightowl's suggestion that you consider an Instapot. A lot of digitally run capabilities mean it handles smaller and larger quantities well , has a lot of programming capabilities for both slow cooking and pressure cooking, and is about as fire-and-forget convenient as you can get these days. Many slow-cooker recipes work very well in pressure cookers and take a small fraction of the time for equally good results. So, say you did not feel like or get around to prepping and loading the slow-cooker before going to work in the morning? Put your ingredients in the pot when you get home, run it in pressure cooker mode, and your beans are done in half-an-hour or so.

Also, the reports seem to be that the Instapot's "keep warm" function works better than the warming functions on many of the newer slow-cookers. For example, I have a 30 year-old rice cooker that can hold cooked rice at around 150°F all day long but the the "warm function" on my Crockpot "Smartpot" seems to slowly ramp up to a bit over 200° which is a slow boil at my altitude. I recall ( but cannot immediately find) a longish thread on this over on chowhound a couple of years ago.

I have the InstantPot and love it. I am not one that cooked much at all but I do love this. I actually now cook a couple of times a week. Once in a while I did use my crock pot but couldn't stand to smell food cooking all day long. Have't tried using the instant pot yet to slow cook but also haven't had the desire to do so. Everything cooks so fast and comes out delicious in the pressure cooker.

The slow cooker recipe book I have assumes a 5 qt cooker. I had a 7 qt and found that things didn't quite work right so I bought a smaller unit. Didn't need the larger size just for myself.

I have this one that has 2, 4 and 6 quart bowls that fit into a base, so you can choose the best size for the recipe.

https://www.hamiltonbeach.com/3-in-one-slow-cookers.html

When I needed to replace my vintage West Bend bean pot style 6 qt (more than 30 yrs old), I was tempted by and bought the Fagor Multi Purpose cooker. A happy decision, I love it - and appreciate it more than any other small appliance in my kitchen. It browns nicely, pressure cooks flawlessly (2 pressures), slow cooks (2 temps), will serve as a rice cooker too. The Fagor is 6 quart, it's what I'd been used to. The silicon seal is a dream.

Nesco had essentially the same appliance, but I was still in my old kitchen then and the Fagor was 1" smaller in diameter, meaning I could put in on a shelf and still close my cupboard door ;)

Another consideration no one mentioned yet is cleaning. I prefer slow cookers with removable insides for ease of cleaning. I have had several over the years and found that the ceramic-looking type are much easier to clean than the stainless. However they are heavy. Mine are 5-6 qt. ovals (much more versatile than round). For cleaning reasons, I would not want anything larger. I find that it does not need to be filled very high. However, I do not like to fill it much more than half full or I need to rotate/mix during cooking because the top part does not get as hot and hence does not cook as fast. All of slow cookers I have used have been crock pot or similar brand.