Something like an intake air temperature sensor requires practically no current at all. Current is what makes the wire heat up. Thicker wires can handle more current without heating up. So in your application, the smallest wire you can find will be sufficient. But if you want some concrete answer, buy something like 1.0 mm² wire as it is widely available in car parts stores and you can use it in future applications too. A 1.0 mm² wire will handle a current of approximately 10 amperes and is thus totally an overkill for the intake air temperature. But it will surely provide some added mechanical strength compared to a thinner wire.
Then, you say that you would be soldering the wires together. Wrong... You shall never solder anything except PCB components and connectors onto the PCB. Anything with no strain relief/cable clamp situated immediately after the soldered part is susceptible for fracturing at the location where the solder ends. As solder in essence is not flexible (compared to a copper wire), any flexing of the wire near the soldered location will crack the joint and cause the wire to fail - intermittently at first and with time a total loss of conduction. Okay yeah sure, soldering the wire can mean that it will take many years for it to fail. So if you don't intend keeping the car for say more than 5 years, go ahead and solder it, it probably won't matter. But if there is the slightest possibility of retaining the car for 10+ years, then, you should do a proper job in the first place.
So what is the proper way to do it? A crimp connector. Notice that you need the PROPER crimping pliers to create a good joint. If you will, you may also use a heat shrink sock placed onto the crimp connector to avoid water ingress. Also one alternative is a version of the crimp connector with an embedded heat shrink (shown on the latter image below). First, you crimp the connector and afterwards apply the heat gun to shrink the plastic. The heat shrink will provide additional mechanical strength and protection from water. If you really want to go over the top, you will squirt some, e.g., plastics compatible grease between the crimp connector and the heat shrink sock before shrinking the sock. This can be done using an injection syringe. The grease will protect the crimp connector and wires inside from any corrosion. After the grease has been inserted you will shrink the plastic onto it to seal the grease in using a heat gun - you shall never use the soldering iron to shrink heat shrink socks as any plastic applied to the tip of a soldering iron will void the coating (ON THE TIP) immediately and you can dispose of that soldering iron tip.
