Have you tried H.264? Most people delivering digital copies use that.
There is no way to get a decent 1920X1080 video into 2G when the duration is 51 minutes.
The bitrate would have to be so low that the image quality would suffer terribly.
Do the math. If you put it out there at a reasonable 25Mb/s you would need a file size of almost 10G.
Divide that by 5 to get 2G and your 25Mb/s becomes 5Mb/s. I suppose you could try that. If it was free you might get away with it. But if you want to sell it, people are going to expect a better image. I know I would.
But there you have it. Use H.264 as Kevin suggests, and set it for 5Mb/s. Export it and see what you think.