The stress concerned is actually the steel stress. Nothing to do on the bearing stress. Bearing stress is part of the prestressing specialist design.
To prevent cracks the stress in the reinforcement should be limited to 150MPa to 250MPa and the area is the steel area – not the concrete (blister) area.
As you can see from the calculation, the reinforcement areas do not have the size of the blister (concrete) in the calculations. In fact, if you increase the size of the blister, the angle may be greater and therefore the tie-back force would be larger. Thus, having a larger blister would increase the reinforcement requirement. Thus it is of no use to simply just increase the blister (concrete) size.