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The minimum size bar used to support a post-tensioned beam is a #4 bar. This specification ensures that the tensioning forces exerted on the beam can be effectively managed while providing adequate anchorage and reinforcement.

Using a bar of this size allows for sufficient stress distribution throughout the beam, which is critical in maintaining structural integrity under load. A #4 bar typically provides a balance between strength and flexibility, which is essential when dealing with post-tensioned structures that are subjected to high stress levels during and after the tensioning process.

The use of larger bars, such as #6 or #8, may offer additional strength but is often not necessary for the minimum reinforcement requirements and might pose challenges such as increased weight and handling difficulties during construction. On the other hand, using a smaller bar, like a #2, would not provide adequate support for the loads expected in a post-tensioned beam setting, risking structural failure.

Thus, the choice of a #4 bar establishes a standard that meets both safety and performance needs in post-tensioned beam design.