I've been working in this scene of mine and I don't know at what time proportional stopped working as it should. However after some basic debugging I think I know where to look. I've attached my scene where I only have a sphere left in the scene.

To test the problem;
1. Open my attached scene.
2. Go into edit mode.
3. Activate Proportional Edit Mode.
4. Move one vertex and try to resize the influence area of proportional edit mode with mouse scroll wheel, the circle doesn't appear nor does the other vertices become affected.

It does work to set the Proportional Size with F6 afterwards but it doesn't solve the actual problem.

I've opened the python console in Report mode and it seems to echo a proportional_size with weird numbers and I don't know how to get it back to normal.

A suggestion for a fix would be to check if the proportional_size number is within reasonable limits when using it, otherwise reset back to a maximum or minimum value.

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Specs: Windows 7 64-bit English, Blender 2.55.0 r32738 (site release), ATI Radeon 5830