You can find a lot of the relevant info in the Google Groups FAQ.
For example, the maximum group size is dependent on the type of Google Group you have:
With Google Apps, a group can contain up to 100 members. With Google Apps for Business and Education, there's no limit to the number of members a group can have. However, if the group contains external members, note that there is a limit to the number of messages you can send to external recipients per day (see next question).
And attachment limits:
Yes, the maximum size limit for messages sent to a group is 25 MB,
including attachments (the normal Gmail limit). Group owners or
managers can edit specific groups to set lower size limits.
More info about messages and attachments can be found on this Google message FAQ.