I have searched a lot. I can say almost for 2 months. But I couldn't find any document about this topic.
Which one is smaller in terms of footprint, Windows Embedded 7 or 8 or Windows Embedded Compact?
Which one of these two requires less RAM and storage memory? How much RAM and disk space is required for the smaller one?
How the old Windows CE 6.0 can be compared to these two?
Windows Embedded 7 Standard is already covered here.
Windows Embedded Compact is by far the smaller of the two in terms of system requirements and footprint. Depending on the components you select when building your operating system, you can reduce image size, and thus storage space requirements, to 500 KB. I haven't been able to find any official information on RAM, but based on experience I wouldn't be surprised if you could get by on 8 MB or less, again depending on the components included in the image.
Windows CE 6.0 is comparable to Windows Embedded Compact; Windows Embedded Compact 7.0 is really just Windows CE 7.0 under a different name.
If targeting even smaller systems you might want to take a look at the .NET Micro Framework, which has a memory footprint of only 300 KB.