How do I determine the size of a video I want to forword in Apple Mail

You will have to save the video attachment to your local drive by right/control-clicking on it to either Save Attachment… or Save to Downloads folder. Then, you can do the following to gauge size and duration.

Click on the video in the Finder and press command+i to Get Info. The size of the video is reported under General, and the duration is reported in the More info section.

Videos are typically too large to send by email. Use a cloud service as an alternative.

You are welcome.