I have created WatchKit extension for my app. Both share a common app group container to store and fetch data files.
What is maximum storage limit for that app group container shared between my app and its extensions?
I believe that there is no special size limit for app groups. (At least, I could not find any information about this in Apple's documentation.)
The amount of data that can be stored is, however, limited by the amount of available memory on your "device". And that "device" seems to be your iPhone (and not your Apple Watch) since the documentation states:
WatchKit settings must be stored in a shared group container that is accessible to both your iOS app and your WatchKit extension. Because the WatchKit settings bundle resides in your iOS app, the system writes preference values to the iOS app’s container by default.