your post is vague. Here are some thoughts: gmail and outlook automatically save to sent folder. if you have thunderbird set to do that, I suggest UNticking it to eliminate saving twice. that may resolve your issue.

If you don't need to access sent messages on other devices, you could set the target for sent mail to Local Folders (if sent messages aren't automatically saved). To save space, define a filter as in the attached picture and run it manually on the folder with sent messages.

thank you @david, the tip about gmail saving automaticall to Sent is great. I can also confirm that this applies to yahoo. But it does not apply to gmx.

I can't understand how my question is vague: Can Thunderbird ask me before saving large emails (e.g. with large attachments) to Sent?

I guess the answer is no. Though it would be very helpful if it did because a lot of bandwidth is wasted for sending an email+attachments "twice" especially since most of the times I erase the copy in the Sent folder because I already have the attachment locally.

Anyway, your tip is useful.

@sfhowes, thank you but I need to see in other devices, but I also have it saving to local folders as well. So your filtering advice is useful. I wish there was such a filter for when sending email too.

I mean it has this "did you mean to attach attachments? because you mention attachments in your message body". Would it be possible to have other such triggers on sending an email which could cancel saving to Sent?