Hey @Ranettenyc, thanks for reaching out to us here!
To compress a video you will need to use a third party app as Dropbox doesn't currently have a feature to compress files before sending them.
What I can suggest is to compress the file before uploading it to Dropbox and then send the attachment through the mail.
Otherwise, maybe using a shared link for the file you want to send would be useful for you here? You can create a shared link for the content you want to send from Dropbox and you'll only need to paste the link in your email.
I hope this info helps, let me know if you have any questions!