I have encoded few movies using this fantastic encoder, but I wish to know how to decide about the movie pixels. OK it's logical to use lower sizes for lower bitrates, but it is still specific for each movie (like movie size, different motion intensity, wide screen or 4:3, quality etc.).

Is the only way to find out - experimenting?

Yes
For differend movies is differend situation, not only hen using mencoder.
Some movies looks beter at high resolutions even with low bitrate, and some even with small resolution and high bitrate are crapp (thus you have to use 2 or more CDs).
Simply makie sample of movie with SAMPLER at end of the script , and you'll have preview of movie quality. That's my easy way.

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Go for SECAM =)

It was easy with tmpgenc (tok and calcumatic) and even with cce...

I am not sure about the samples because some high-action scenes may not be included... And I prefer to use internal ffw scripts...