Because this is a rather massive undertaking I have been doing a bit of panicked knitting myself and experimenting with different sized needles, so I can share some gauge-based wisdom with you now in case you want to join in (yes, please want to) :)
So far I've done a few squares with 4mm needles, a few with 5.5mm ones and a few with 8mm ones.
needles, for reference
I have worked out that to make a 10x10cm square with 4mm needles, I need to start with 20 stitches. For 5mm needles I started with 16 and for 8mm ones I started with 12. This will vary slightly depending on how tight you like to make your stitches, but since knitting is stretchy I don't mind too much if the squares are a couple of millimetres smaller or bigger than 10cm. Also, I have no idea how many rows to do. I've just been knitting away until it's 10cm long. I'm so precise and accurate about things.
l-r: 4mm needles, 5.5mm, 8mm
So the above picture illustrates the different gauges produced with different sized needles. 4mm needles will make a tighter, more rigid, thicker fabric with less gaps between stitches. 8mm needles make a loose, floppy, gappy fabric. Both are fine for this project, so do whatever you want. Bigger needles mean you finish squares quicker, and also use less yarn: which means a) you can squeeze more squares outta that yarn ball, and b) the finished banner won't be quite as heavy. I think a mixture of different gauges will look good and patchworky, so like I say: do what you want :)
Same goes for stitch pattern. I've used garter stitch mostly but you could do some stocking, moss, lace, cable, or whatever makes you feel happy.
I have made one crocheted square so far: a granny square made of double crochet stitches, four rounds, with a 4mm hook. Again, yours might be different. The only stipulation is that they are 10x10cm!
Oh yeah, the only OTHER stipulation is that all squares are red or white. I need a lot of both, so do whichever. Red can mean scarlet, crimson, vermilion, puce, magenta, maroon, burgundy or whatever reddish shade you've got lurking in that stash bag behind the couch. White obviously doesn't come in that many shades, but I will accept cream or very pale grey as I think it'll look good.
err.... can't think of anything else right now. If you are coming to the first workshop tomorrow, please bring your own needles/hook and yarn if you have them - I have a few bits but I'm scared we'll run out. Also please bring any squares you have made already! I'm delighted that so many people have expressed an interest in taking part. So we have to get this done now crafters.
Maybe see you tomorrow, maybe not - but happy and rebellious crafting to you! you're the best xx