I use 10AWG with silicone insulation for my SK3 6374’s and it’s overkill. I’m sure I could run 12AWG and be fine.
Generally in these situations you reference an “ampacity chart”, as the current through the conductor is the driving factor of how beefy it needs to be. Voltage is generally limited by the insulator. e.g. here’s some 22AWG hook up wire rated to 5,000 volts - but it wouldn’t handle any more than 3 amps.
Your requirements of 48V and 1KW boils down to a 20.83A required rating (1000 / 48 = 20.83). Aside from conductor size (strand size, count and bundling really) and material (copper > aluminum), the other factor is how hot it is allowed to get which is limited by the insulation and where the wire is going pass physically.
I would go with stranded 10 or 12 AWG with silicone insulation. Nice and flexible, still well within soldering territory, can get very hot if it needs to, and would handle your 1KW load at 100% duty without uncomfortable/painful heating of the conductor if you put your hand down on it.
I usually go with this product when I’m wiring a new motor https://www.amazon.com/gp/product/B0756TDTDM
edit: I wouldn’t worry about voltage drop or anything like that for such a short run. You’re gravy.