Is there any command line switch or workaround for start chrome with some custom zoom?
Do you mean always? Or sometimes (like from a particular shortcut on the desktop)?
You can set a default viewing zoom level in the Chrome settings page (chrome://chrome/settings/), by clicking on Show advanced settings..., and scrolling down to the Web content section. This, however, will apply to every instance of Chrome, and not a single one as a command line option would.
If you just want to temporarily set zoom for testing purposes you can set the zoom of the body in your css. That is essentially all chrome does anyway.
This would zoom the page to 50%.
Or if you want to do it for every page chrome opens you can change the default zoom in Settings click the Show advanced settings... link and look under the Window Contents section. This can be annoying though because every page you open will be effected.