Vertical or horizontal venting for a 36" range hood with a 3 speed 630 CFM blower
A larger pipe will certainly increase the flow as will shorter pipe lengths. Fittings are expressed in equivalent pipe length. A 90 degree elbow is typically 5 feet and a 45 degree elbow is typically 2.5 feet. So you can add up your total length and add 5 feet for each elbow to get your total equivalent length. The smaller that number is the better. The only issue you will have with increasing the pipe size is you will decrease the velocity and that won’t remove solids like grease as well but that is seldom an issue with residential hoods. On another note, all hood manufacturers lie about their CFM. 630 CFM through a 6” pipe would be a ridiculous amount of static pressure and you wouldn’t be able to stand the noise or the electrical bill. I would guess when all is said and done you will be closer to 200 CFM but anything you can do to minimize the total equivalent length of duct work or make the duct bigger will help in maximizing the output.