What is the maximum file size I can send via iMessage?

I've had a look online, and there are lots of conflicting reports. Some sources report you you can send a maximum file size of 3.5 MB when sharing media with non-Apple devices, but 2 GB with another iPhone via iMessage. However, other reports seem to suggest it's always a max of 100 MB, regardless of device.

Which parts of the above are correct? Based on this User Guide article, the answer is "The size limit of attachments is determined by your service provider", but that's not very helpful unfortunately with the conflicting information out there!

So my question is: What is the maximum file size I can send via iMessage, either to a non-Apple device, or an iPhone?