If you are using Hiwin 20 long blocks/ or even short/ , 200mm between both ends from outside measured is good for your machine. 260mm is desirable for serious build and i would say ideal, and 300mm for OTT and heavy gantry and really strong and large machine. So basically with Hiwin long blocks they will be almost touching each other .

The idea is the bit to be in that distance , looked from side. In other words the spindle overhang. But on a machine like yours even if not, its not a big deal, as bearings are much more rated than forces involved.