As you may already know you can send quite a large video if it is sent to an email address but there are more stringent restrictions if you want to send a video as an MMS message.
For messages with a video attached it appears that the maximum size is 1,175,000 Bytes and also only 60 seconds long maximum. Not to worry, it is easy to do as follows;
Bring up your camera. Touch the camera icon (upper right corner) so it becomes the "video camera" ICON. Then on the left-middle side of the screen touch the white triangle and then touch the second ICON from the top (looks like a video camera with a reel). On the screen that comes up touch "Limit for MMS." Back on the right side touch the red dot and your video will start. Let it the video run and at the same time read further.
Now here is how you comply with the size 1,175 Bytes and the 60 second rule. As your video is running notice at the top-left of the screen the seconds are ticking away and at the bottom-left of the screen the number of bytes are also ticking away. Now, here is why I said not to worry...it's because the video will stop automatically, whether you want it to or not, at 60 seconds. The only thing you have to do is make sure you have included everything you wanted during this 60 second video. By the way, to stop the video before it reaches 60 seconds touch the "Black Colored Square Box" that is just below the Red Dot you previously pressed to start the video.
After that you can touch the "Image Viewer" which now is where the the "Black Colored Square Box" was before you pressed it to stop the video before the 60 second maximum OR the video stopped at the 60 second mark. Also, in a couple of minutes your video will show up in your Gallery.