With kdenlive I can choose H.264 with fixed bit rate (for example 18000 kbps). The resulting file is huge.

With avidemux I can optimize the size with variable bitrate. Is there a way to do the same with kdenlive?
So the resulting file has a good quality with a smaller size.

Thanks.

not sure what you're asking. if you want a smaller file then chose a smaller bitrate. if you want a custom render profile then you can add one by clicking on a profile you'd like to copy from, give it a new name, and change whatever you want. in this picture, notice the piece of paper with a green plus (+), just click on the profile you want to copy, and then click on the plus symbol, go from there. good luck

http://www.kdenlive.org/sites/default/files/missing_codec2.jpeg

I think there is no such possibility yet.
However as far as I understand 2 pass rendering it should make this a bit better. The video will be rendered two times. During first pass analysis is done which then is used to optimize the final render. So the bitrate will be lower at times where it is not needed that high and higher at complex parts.