It's a good idea to plan ahead and think about how you'll be powering your projects on and off. Sometimes it's totally OK to just toss a power switch in, but if you plan it, you might avoid potential road blocks.
When I first started making Adafruit projects, I'd just toss a slide switch into a 3D printed enclosure and call it a day – I quickly found that to be problematic, only because it's a pain to open up a project just to turn it off (especially when there's screws or glue involved!).
This depends on the project. The length of wiring should be determined by the size of the project. Maybe the switch needs to be a certain distance away from the battery. Or maybe the switch needs to be further away from the circuit. Think about these factors to figure out how long the wires need to be. In some cases, for those really small projects, maybe it needs to be the shortest as possible.
Picking the best location for the power switch is just as important as the length of wire it needs to be. Will the power switch be easy to get to? It shouldn't be a pain to reach behind your project just to power it off. Does the location of the power switch make sense? Is it easy to accidentally power it on/off? Maybe it's better on the side than the back. Think about this so you can allocate real estate for the power switch.
It does not have to be this type of slide switch. There's lots of different types and styles of power switches. It could be a bigger push button type, an arcade button or maybe even a rocker switch – just make sure its a "latching" type, and not the "momentary" type. Depending on the size of your project, you'll need to pick which one best fits your project.
Depending on your powering needs, you may need to get a beefier switch that can support a higher amount of amps/voltage. It's a good idea to check the datasheet of your power switch to ensure it's capable of handling your projects needs. Most of my projects tend to be around 5V, so they're very light on power – the breadboard friendly SPDT slide switch is my "goto" component for power switching.