Well, I think Yoda was talking that size didnt matter to a force user
sure, size isnt always the defining factor, but if Kenobi doesnt have to force to stave off the fatigue of Banes Djem So, then i sont think he would last all that long. Bane is, to put it simply, huge
He is a 2 meter tall miner that was mining stuff that is more than likely heavier than steel. To put in an example, he bit off a mans thumb, albeit possibly unconciously using the force to augment his jaw strength
Soresu is a defensive lightsaber form and Obi-Wan is the absolutely undisputed master of Soresu. Bane may look and stocky like Brock Lesnar and utilize Djem So like Anakin, but Obi-Wan has a definite possibilty of winning as Lucien stated. Kenobi defeated a blood lusted Anakin who practiced Djem So and he and Anakin sparred many times so Obi-Wan would be very familiar with the form.