I was wondering how big a document (picture, voice, video) can be "iMessaged" from an iPhone. Also, would the iMessage data size be any different if you are iMessaging from within Mountain Lion.
Extracted from [Mountain Lion's Features] page:
Send large attachments: Send high-quality photos, full HD video, or documents. iMessage allows attachments of up to 100MB.
The max file attachment size for the iMessage and Business Chat services is 100 MB. Platform (iOS, macOS) doesn't matter in this case. Both channels are IP-based, not carrier-based, so it's just WiFi or cellular data.