Glow Blogs is not designed for hosting video. It is better, from a performance point of view, to upload video to a video hosting service and embed the video on your blog.
That way the video hosting service will deal with encoding, providing different versions for different devices etc.
If you upload to glow blogs the behaviour of the videos is dependent on the file you upload. (for example .mov files do not play in Google Chrome).
It may be that you are limited for choice or do not want to use external services, Blogs may then be your best option. If so you should upload the video in a suitable format, mp4 seems to be widely playable and you are limited to the size of video you can upload.
File size upload is limited to 50MB on glow blogs. The video above shows ghow to use Handbrake, a free video transcoder to reduce the file size of your video.