How can I find out the size of a video file on my iPhone 6 camera roll?

The Shortcuts app made this actually rather easy nowadays. Configure it as an extension for the share sheet, then activate it in any video (or photo) within Photos.

PS: Shortcuts is part of recent versions of iOS. For first time users it might help to have a look at the Shortcut Manual.

Another 3rd party app option is the Google Photos application. You can click a photo or video and select 'info'. It doesn't require initiating an upload and then canceling it.

I wish this was built into the native iOS Photos app, but at least Google Photos is free and comes from a reputable vendor.

Both OneDrive and Dropbox etc. show you the file size as soon as you start to upload the video. Once you've glanced at the video size, there's nothing stopping you from cancelling the upload right away so you dont spend unnecessary data.

An option that needs no upload is the View Exif app extension - it will process the video file on your phone/iPad and tell you the size of the video and no need to leverage another service.