Some of the older Leicas have very small viewfinder windows. I think you are better off asking this in the RF section.
BTW, what is the attracting feature of using a RF over another system? (ie. SLR, DSLR, MILC, serious compacts)
With rangefinders, you need to be comfortable with the focusing system as well as with manual focusing.
Many folks use pre-focusing and zone focus to get over the need to MF fast or eliminate the need of focusing altogether.