by watercooling design, do you mean the CLC design shirt? I'm not even seeing a medium listed for that one, so chances of it coming back into stock seem slim. I don't know how big the T-shirts tend to be for a certain size, (I think how big a certain size can vary by manufacture) But a large might also work if it increases the range of options (I think that t-shirts can be a bit big without much of a problem)
as for how many 5XL shirts they sell, I don't know, though some might like the idea of an oversized t-shirt, so there could be sales of that beyond just people big enough for them to fit.