AutoGK automatically calculates bitrate based on options you set in the main window. Also aspect ratio is calculated automatically if you choose a custom file size.
You can see some advanced options pressing CTRL + F9.
In the main window there is a large button named Advanced Options. Click there and select fixed width and then 708 as value. This should set size to the value you want but, again, bitrate is determined automatically by AutoGK based on other options you set previously.
If you want a high bitrate, I suggest to use the option "Target Quality (in percentage)", setting a high value. 100% is not a good idea, because it will create a huge file (probably larger than the original dvd). Usually 80% is a high enough value, but the best option must be selected doing some tests and watching results in your television, to be sure that video is shown good. If you want a lower bitrate, then select a lower value, 70% for example. A too low value will make video appear ugly, so again you must find the right value trying some options by yourself.
Bear in mind that even if you find a good target quality value for that specific movie, this value is not the best option for every movie, because results are related to the quality of the source DVD.
If the original DVD has a high quality, you can set a low target quality and still obtain a good result, but if the original DVD already has a low quality, compressing it into avi will further cause a degradation, so you should insert there a higher value.