Was just wondering if someone could clarify something for me please

If you have a look at the photo below you will see that in my lounge are I have a distance of 5 m between two walls but in my bedroom area with the skillon roof layout being built the rafters will sit on timber in between the front and back wall at a distance of say 3.6

Does this mean I dont have to use timber size that will span 5 m etc 200 x 48 Lvl or could I use something smaller or even up the spacing from 900 to 1200 on the rafters


and span tables