This is currently a known limitation of the Shader Graph, which does not offer a way to set the point size. On some graphics APIs, including Metal, rendering points without setting the point size is undefined behavior. See "Semantics used by Shaders" at https://docs.unity3d.com/Manual/SL-PlatformDifferences.html for additional information.
While support may be added in the future, here are some ideas for workarounds in the meantime:
- You could use the Shader Graph to generate HLSL code via the "View generated shader" option. You could then patch this code to set the PSIZE attribute, and use this shader for your material. Note that this step needs to be repeated whenever Shader Graph or Unity or updated, or when the Shader Graph is modified.
- You could try rendering instanced triangles or quads instead of points