Just to put things in perspective. Large prints can be made from modest size images. I have seen decent A2 prints from 6MP APS-C cameras, like the original 10D.
This picture hangs on my wall printed on nearly A2 size paper. It has a few artifacts, but looks quite OK if viewed at arms length. It was shot with Minolta Dynax 7D (6MP APS-C).
I also have a 70x100 cm print on my wall from a Sony Alpha 100 (10 MP APS-C). It's OK if viewed at 100 cm or more.
So I certainly prefer the Alpha 900 with it's 24 MP, but if the image is compelling, smaller formats will do fine. But I also feel, I wish I had the camera I have today when I took that picture.
The Iceland image is available here: http://echophoto.dnsalias.net/ekr/images/Samples/Iceland_Sample2.jpg
, this license applies: http://echophoto.dnsalias.net/info/licensing/cc_license.html
Yes you're right that 5D pixels are considerably sharper than 7D, so one could probably upscale more there. I have personally more experience with 7D prints than with 5D prints so far.
Just for fun, here's an example of a picture from the 7D, shot with TS-E 24mm II at f/8. There's some tilt, maybe a little shift too, don't remember for sure. According to my experience this is as sharp as it gets on the 7D with this lens. 100% crop without sharpening to the left, 100% crop with mild deconvolution sharpening, and then the full image to see from where the crop is taken. For a high ppi print (300-400) I would probably sharpen a bit more, with some halo even, and for a low ppi print (less than 200) I would probably not sharpen at all to avoid pixelation around high contrast edges, but it depends on print technology and what the printer driver does too.
The 7D has 4.16 um pixels, I would not really mind if fullframe is eventually pushed this far (46 megapixels).