How do I see the whole subject line after opening the email?

[Ginaccc]

I have a boss that tends to put a lot of information in the subject line of an email.  I can't see the whole subject line unless I hit reply.  Is there a way, when I open the original email, to see the whole subject line? I looked around in setting but couldn't seem to find a way.

[mikeone]

In message view, just tap on the subject / adress lines to expand the "header portion "
Tapping again will collapse the expanded header.

AquaMail retains the last state.

[StR]

If you tap on the header portion (there is a down-arrow on the right side), - it will expand and will show the Subject, as well as detailed "To:" and "From:" headers.
I am not sure if there is a limit there, - but I see at least 3 lines of Subject.

If you still hit some limit, - you have an option of changing your boss. 
The alternative would be to educate him/her on the "email etiquette".  And you can  that in a way that would not make him/her mad (or personally offended): Suggest that your team would benefit from learning good practices of e-mail communication, and offer a link to one of the numerous websites about "email etiquette", e.g. https://www.businessemailetiquette.com/business-e-mail-etiquette-basics/  .
Actually, this one might be even better for your needs:
https://www.lifewire.com/how-to-write-a-good-email-subject-1173268

[Ginaccc]

Hi, thanks for the replies. I never knew it would stay open after I clicked on it, that's awesome, thanks!

I have tried to tell her about emails, I mean, she put the date of a conference call at the end of the long subject line and didn't bother to put it in the actual email. Lol grrr

Thanks!