The size of the Hot water cylinder is totaly non relevant to the water pressure, but raising the water tank as high as is practical will increase the water pressure. A noticeable increase I am not sure, that depends how much more head ( height you can raise it by). For every 1meter you raise it, you will benifit by 0.1 bar of pressure increase. So if you can only raise it by 1 meter then you will not realy notice it. But if you can build a sturdy platform between the roof rafters and raise it by 3 to 5 meters or more from its current location, then yes you will notice a difference. Just remember to increase the expansion pipe aswell to allow for the greater volume of water and the hot water expansion.