O365 Group Mailbox Reports?

The only reason you would need an EO2 license for shared/room mailbox is if you have enabled hold, and if you are applying a license anyway, you should just make it a user mailbox.

Onto the Groups question, you can simply use the "calculated property" method:

Get-UnifiedGroup  | select DisplayName,Recipient*,@{n="Size";e={(Get-MailboxStatistics $_.Identity).TotalItemSize}}

DisplayName RecipientType                  RecipientTypeDetails Size
----------- -------------                  -------------------- ----
Unified     MailUniversalDistributionGroup GroupMailbox         1.314 MB (1,377,400 bytes)
First group MailUniversalDistributionGroup GroupMailbox         7.702 MB (8,076,248 bytes)

You can add other properties as needed, but in general is better to write a proper script instead of using oneliners and the pipeline.