I don't recall exactly, but I vaguely remember that significant successes on spirit summonings - or maybe it was just Great Form spirits - could be used to add on powers, including Reach bonuses. Don't quote me.
However, typical game play puts both a spirit's size at 1' per point of force. A Force 3 spirit would be 3' tall, while a whopping F14 would be 14' tall. The thing with spirits is that they can (IMO should) have a certain link between their appearance and their power; a Force 3 spirit doesn't have to be 3' tall, it could be 30' tall, but at that great height it's going to look wispy and weak. Conversely a F14 spirit might be only 4' tall, but it is going to seem more dense, more vibrant, more real that reality around it - practically vibrating with contained power.
Oddly, I feel the same way about matrix icons - the higher the rating, the bigger it can be - or possess a higher resolution.