Reading the info on RatDVD official website it seems that to gather best quality (for unspecified file size) it is better the â€œConstant quality encodingâ€.
How are the quality settings working?
The quality settings depend on encoding mode and are different for "Constant Quality" and "Constant Average Bitrate".
In "Constant Quality" mode the video encoder uses an adaptive quantization scheme which uses a psycho visual perceptive model so that it quantizes the most where you're the least likely to see it. The "average quant" generated by this model is what is wired to the quality setting parameter (inversed to generate an intuitive user experience). The closer to 120 (The closer the average quant is to 0) the more accurate will the pictures be and the better it'll look obviously the bitrate will climb too. It's a constant quality encoder and this means that it'll carry through on the quality setting no matter what bitrate that might produce. Constant quality transcoding is advantagous quality wise over rate control schemes.
In "Constant Average Bitrate" mode you can specify the average video bitrate and with it the size of the resulting ratDVD. In this mode the bitrate of the encoding is constantly controlled and if the bitrate needed is different than the target bitrate, it is slowly adjusted to meet the average bitrate target and with it the predefined size. This adjustment is made slowly overtime to make sure that scenes that need a higher bitrate will get it. However, this slow adaption to unknown video material can lead to not meeting the target size exatly in case of special video content or very short video material (which shoudln't happen if you convert from DVD).
Quality wise it is suggested to use "Constant Quality" encoding but in certain cases (e.g. you want to put a DVD an a memory stick) you may want to use "Constant Qaulity" to meet a given size.
Can the file size of the .ratDVD file be estimated in Constant quality encoding?
What is the typical file size/compression level from DVD as source?
No! If you want to estimate the destination size you have to use "Constant Average Bitrate" encoding.
Instant "Constant Quality" mode every frame in the video gets the bitrate it needs. This ensures the highest possible quality but means that the size of the converted video greatly depends on the video material. Interestingly there is almost no relation to the original DVD size - only the quantity of frames in the original is of some meaning.
"Constant Quality" is the default transcoding method because I am a fan of the highest possible quality and compared to DivX, XviD, etc. in ratDVD there is no need to fit the file more or less exactly to a CD - because when it gets played back in a DVD player it gets blown up to a DVD anyways.