Is there any way to improve the quality of snapshot and enlarge the size?

Use a screencap program instead of the game's crappy camera.

I use Fraps. It's free and takes nice large screenshots. You will probably want the no red pause line hack if you use Fraps or some other screenshot program. http://www.insimenator.org/index.php/topic,62217.0.html

Resurrecting this old thread because it has not been answered, and we have not had any answers in this Stories section in a while and that's a shame

Yes you can have better C shots in CS, you need to modify three files:
-in Documents Config/UserProps.xml
-in Program Files TSData/Res/Config/globalProps.xml
-in Program Files TSData/Res/CSConfig/globalProps.xml

Search for snapshotPictureSize and snapshotPictureQuality, and change the value to 3, like it is in TS2 on max quality and 600x450 resolution.
Don't forget to always backup the original files before modifying them.