I can not give you a rough estimate on the size of the file resulting for the conversion process because it varies from one DVD to another.
However, you might be able to do a close calculation like this:
1- Convert 1 chapter from a DVD.
2- Look at its size and multiply that by the number of chapters you have.
Make sure you set the same video and audio compressors for the big DVD to be the same as the small chapter.