One can make an analytical solution to the problem without knowing either loads or spans. The important difference between 45x220, 45x195, and 45x170 in this case is the difference in moment of inertia. 45x195 has a moment of inertia that is about 70%, and 45x170 one that is about 50% of what applies for 45x220. This means you can replace 45x220 with 45x195 if you reduce the load to 70% (c/c 900) and with 45x170 if you reduce the load to 50% (c/c 600). All under the assumption that the wood has the same strength class.