I don't think the CoCo I could be expanded beyond 32k since that's as many address lines as they brought out to the bus. When I worked for Alpha Products/Colorware, we sold a MacPaint clone for the CoCo2 that required 64k.

CoCo3 came with 128k out the box though.

I could easily be misremembering. I know it was a CoCo I, so it must have been 32k instead.