HD .ts > Avi ... end file size?

[NotSoEasy]

I know I can pick bitrate and file size and audio bitrate.
I am wondering how does one determine the end file size.I am always confronted with having to choose the size in MB ..... well how do I know what to pick?
Say I chose bitrate off 500 and the audio is 128.I can pick 700MB in size if I want.But is that going to be right?
Say I chose that same size (700MB) but change bitrates.... still same 700MB file but different bitrates.
How do you know what the file size should be.Say my .TS clip is 650MB
Should the XviD of that be 400MB? 300MB?
When loading up bitrates in bitrate calculators, you can change those bitrates, but they dont make a selected file size for you.

[Baldrick]

Yes, you can set the output size with my calculator www.videohelp.com/calc , select DivX and click advanced to set custom output size in MB

The source file size does not matter, only the runtime and video and audio bitrate.

[NotSoEasy]

But how can you know the size is right?
What if I make it a 200MB file ? I just went from a 650MB file down to 200MB
Are the video and audio bitrates going to determine the playback outcome?

What Im trying to figure out is how low do you go before quality is going to be an issue.
I wouldnt think going from say a 4GB .ts to 350MB Avi is going to give me a good result no matter what bitrate I use.