After fixing bug #157506 it became evident that the size of the mask buffer allocated for VBR brushes is slightly larger than necessary. For a e.g. 0.5 radius brush the allocated mask buffer is 3x3 pixels, causing a 1 pixel-in-diameter-brush to be presented as having a 3x3 size in the Brushes dialog.

The problem lies in the way the mask buffer size is allocated for generated
brushes (code from gimp_brush_generated_get_half_size()):

    case GIMP_BRUSH_GENERATED_CIRCLE:
      *half_width  = ceil (sqrt (x_axis.x * x_axis.x + y_axis.x * y_axis.x));
      *half_height = ceil (sqrt (x_axis.y * x_axis.y + y_axis.y * y_axis.y));
      break;

For a 0.5 radius brush, *half_width and *half_height will both be 1 on the
break, causing a 3x3 mask buffer to be allocated later on through (code from
gimp_brush_generated_calc()):

  mask = temp_buf_new (half_width  * 2 + 1,
                       half_height * 2 + 1,
                       1, half_width, half_height, NULL);

Since I consider this a minor issue, (I personally doesn't care of the size of
the buffer as long as the brush width is what I want,) I will concentrate on
other 2.4 milestone bugs and let someone else handle this, at least until there
are no other 2.4 bugs left of which I have contributions to make.

Fixing this should involve a single-file patch (for app/core/gimpbrushgenerated.c),
given that the algorithm can be easily modified to handle the smaller buffer
size (of which I'm not sure).