It varies from mod to mod but 26 or 28 gauge is often recommend. I have some of this in a couple of different gauges plus I have a few other types in different gauges like some two conductor 22 gauge. Stranded is generally better/more durable, but solid can be easier to work with (or you know just get good at tinning your stranded wire). Some people like to use ribbon cables and you can get them pretty cheap in a number of gauges off of Amazon as well.