@mehdihasan It looks like the max video size that can be uploaded is 8 MB (EDIT: actually 40 MB, see thread), which may make a pretty short video depending on the video's bit-rate. Right now, it seems the best options is to upload it to another video side (I.E., YouTube) and link to it in your post.

Video is one of the hardest thing to store on a server, so limiting how big they can be was probably the best compromise at the moment so that servers don't get filled out by video files too fast.