How to change the maximum message size in Thunderbird for Android?

There's no setting that limits / configures the size of messages that can be sent.

The only size related setting governs fetched messages. settings > account > fetching mail > fetch messages up to: 32KiB

What element is taking up the bulk of the emails size? The text, inline or attached images, file attachments?

I've tried to recreate your issue by sending an email with a 3MB attachment which sent successfully. I've also forwarded an email containing 1MB of plain text in the email body, which also sent successfully.

The app did have some issues when pasting the 1MB of plain text into the message compose screen. The compose screen just closed itself, possible by crashing.

It doesn't sound like this was an issue for you since you managed to click the send button.

It was a large picture - I was sharing from Samsung Gallery. The email sat in Thunderbird's outbox (and then "Failed to send some messages / Unable to Send message" appeared as a notification).

Thanks.

I tried unsuccessfully to reproduce by sending a large 2.2MB 1920x1080 wallpaper image. The email sent without issue.

Was your experience a one time issue or something that happens with every large image?

There's some reports on github of emails sending successfully, but get stuck in the outbox, generating the failed to send message. See ticket #9230 and #11037.

It sounds like your issue is different as your email didn't actually send.

You could open a new github ticket for your issue, or add to one of the above if you think they reflect your issue.