To solve this question, we need to understand the concept of reactions in a simply supported beam subjected to vertical point loads. Let’s break down the thought process:

1. Understanding Reactions in Beams:
   In structural analysis, a simply supported beam is a common structural element that resists loads applied perpendicular to its axis. It is supported at both ends, typically with a pinned support at one end and a roller support at the other.
2. Reaction Forces in Beams:
   When vertical point loads are applied to such beams, reaction forces develop at the supports. The magnitude of these reaction forces depends on the location and magnitude of the applied loads.
3. Analyzing the Given Options:
   
   • The span length of the beam only: The span length could affect the distribution of the forces but not directly proportional to the length of the reaction vector.
   • The distance between loads: This affects how the loads interact but not how the force vector length is determined.
   • The beam’s deflection at mid-span: Deflection is more related to stiffness and material properties, not directly to reaction force magnitude.
   • The magnitude of reaction force at the support: Indeed, the length of the reaction vector is directly proportional to the magnitude of the reaction force itself.
4. Conclusion:
   The only option that directly explains the proportion of the reaction vector’s length is the magnitude of the reaction force at the support. This is because the graphical representation of reaction forces often involves vector lengths proportional to their magnitudes.

Therefore, the correct answer is: The magnitude of reaction force at the support.