its how big things look through the viewfinder, 1 or 1.1 would mean its no different to real life, while say .25 means things look tiny in viewfinder.
but it will relate to what lenses you use, 28mm or 35mm or so suits a low to mid magnification, while 90 or 135 you want all the magnification you can get,
base length is distance between the main viewfinder window and the secondary one for focusing, more is better, really
i think effective base length combines the 2 or something similer.
if you doing ultrawide angle then you will need a accessory viewfinder, and id probably try and go with a high mag viewfinder for more acurate focus
but rangefinders are really good at middle stuff, middle focal lengths, middle distances, but suck at macro, long lenses etc
personally i like 40mm focal length, and wanted something light so went with leica cl, 40mm vc 1.4