Most on here use a distance of about 200mm for the carriages, not sure if there is any scientific reasoning but 200mm seems a good compromise between compactness and giving a sound footing for the gantry.

The other thing you need to bear in mind is not having your spindle dangling too far in front of the front bearing, keep it as close to the gantry as possible. In an ideal world you'd have it between the bearings but then you'd lose cutting area as the gantry would be wide....