Upload from whom? Does it need conversion? What format will be used to display it?
Upload from a user can't be handled by javascript, since that won't give you access to the user's filesystem to check filesize. You'd need something else, possibly a java applet.
IF conversion is needed, measuring upload from the user will not be enough, since you will also need to deal with conversion time, but once the file is on your server, this should present little trouble.
How you play the video would depend on the format it's in. e.g. VLC might be used to view a wide variety of video formats. In browser viewing is probably most commonly handled by a flash player to view shockwave flash files.