I have a big tiff file and i want to create tiles with gdal_retile.
However I am still concerned about the pyramids levels and the tiles' size.
how can I choose suitable pyramids levels (for example 5, 6 or 18 zoom level) and the pixel size for each of my tiles?
I don't think you can make a sensible tradeoff without more information.
I would suggest looking at the "Well Known Scale Sets" that are defined in the OGC WMTS specification. That will give you something that is easy to work with when you're mixing in other (raster) data.