I use a mix of 30 and 34 gauge. The 30 gauge is great for stuff that's a bit larger, and the 34 gauge is small enough for 0402 components and tight spots. I'd recommend grabbing some bare copper, as well as enameled copper wire of each size. You can use the bare wire for short jumps where there's no risk of anything else making contact, and the enameled wire for longer jumpers or in spots where you're next to a lot of other components.

This is mostly for graphics cards though, so if you're working on larger devices you might want to pick up something like 20 or 24 gauge. It's also important to keep voltage and amperage in mind for the circuit, to make sure your wire can handle the power.

0.8mm is what normally use and 0.3mm when I need to be more in control. https://northridgefix.com/product/best-solder-wire-roll-100g-0-3mm/

Please guys, use metric system, much easier in soldering, especial when components themselves use millimetres