omega.com offers about the most complete and concise information about temperature sensors (RTDs and Thermocouples) of any site on the World Wide Web. They explain and detail just about any aspect of installing and troubleshooting temperature sensors (and other sensors as well, but not as complete as temperature sensors).
Another good source of help for questions like this are your preferred wire provider's sales representative. Any sales representative worth his salary should be able to provide you with recommendations for one or more cables that would suit your application. Be prepared to answer questions like how the wire will be run (closed conduit or open cable tray, indoors or outdoors, etc.
There's just too much we don't know about your site and its needs. The way most RTD monitors work is to measure the resistance of the leads and subtract that value from the total resistance in order to arrive at a proper value of temperature at the location where the RTD is located. Some RTD monitors have a maximum recommended lead length stated in the instruction book provided with the equipment; some don't, but it's worth checking.
Hope this helps!
You could use a stranded copper wire, minimally 075 mm2.
Based on the type of your RTD (I mean 3 points or 4 points) the length of the three/four wires must be the same.
That's kind of a no brainer, should I have said they should be the same size as well?
You only need a 4 wire hookup if you are trying to read to 1 or 2 decimal places
Use the same spec you use for a single pair twisted cable for 4-20 mA only make it 3 conductor.
It's also acceptable to run it into a field junction box and use a 1-1/2 pairs i.e. 3 pairs for 2 RTDs I often do this having a mix of 4-20 mA and RTDs in one home run cable.
It does not matter what you use or what the cable resistance is as long as you do line compensation (some call it temperature compensation but it is actually line compensation) at the final display. Four wire RTD will be the most accurate and also the type I would recommend you use in a long installation like this since you will do double line compensation for it at the display, making the final displayed value very accurate.