I used
\documentclass[twocolumn]{book}
\usepackage[top=1.5cm, bottom=1.5cm, left=2cm, right=2cm]{geometry}
to make A4 paper size book and then join two A4 pages to one A4 page landscape (that created two A5 papers). The fontsize after joining two pages into one was quiet suitable.
I would like to use B5 paper size
\documentclass[b5paper,twocolumn]{book}
but now I would like to specify directly the fontsize, that would be suitable and comparable to the fontsize that would be created by joining two B4 pages into one B4 page.