The brush usually has nothing to do with the size of your pole :
it`s all about the size of the windows,obviously the bigger the window the bigger the brush,there can be windows on the 3rd or 4th story smaller than the ones on the ground floor so it would be best to get a bigger brush anyway to tackle all windows the size is really your choice,i use 2 a 10inch and a 16 inch brush and they both get used on different jobs,hope this helps,Gaz.