how do i select a bit rate that most closely resembles the source videos bit rate to have it come out a similar size file?You set the bitrate to exactly the same as the source bitrate.

The ONLY thing that determines the file size is the bitrate. Resolution doesn't matter. The codec you use doesn't matter. Only bitrate (the "average" bitrate, just to be precise) matters.

There are some "constant quality" codecs (Sony doesn't have these) which determine the bitrate for you, but in the end, even with those, it is the bitrate which determines file size.

If you don't know the bitrate of your video, use MediaInfor or GSpot, both free utilities. They will tell you the bitrate.