The following parameters are defined:
1) Outline: Defines the thickness of the outline. 100% means, it uses the complete area.
2) Smooth Outline: How much of the outline's thickness is used for smoothing. 100% means a mirrored gradient from the center to the inner and outer border of the outline.
3) Outline Opacity: Controls the transparency. 0% means fully tranparent, 100% means fully opaque.
The values are all relative, that might be confusing at first. For example, if you have a 128x128 sprite and set Outline to 50%, then half of the area is occupied by the ellipse. At any angle the ellipse is at both sides of the center, so the thickness will be 32px (32px on both sides = 64px, 64px = 50% of 128).
Likewise, smoothing is calculated. It is a value relative to Outline. So in this example, a value of 25% means 25% of the outline's thickness (which was 32px), or 8px in total. 4px for the outer side smoothing and 4px for the inner side smoothing. In result the ring will be smoothed in from px1-4, full color from px5-28 and smoothed out from px29-32
Based on the description above, if you want a circle (an ellipse would break it) to always have the same pixel thickness, regardless of the size, you'd calculate the proper value:
pixel in percent = (1 / totalpixel) * 200 * n
(sprite is 128x128, outline shall be 1px)
Outline = (1 / 128) * 200 = 1.5625%
(sprite is 30x30, outline shall be 5)
Outline = (1 / 30) * 200 * 5 = 33.333%
(sprite is 200x200, outline shall be 3px)
Outline = (1 / 200) * 200 * 3 = 3%
I didn't implement this, because it only works for squared sprites (because pixels are squared, too). In the last example, we have 3px horizontal/vertical, but if we set the height to 100, we have 3px horizontal and 1.5px vertical.
EDIT: I've accidentally uploaded an earlier developer version This is now corrected. (April 29, GMT 3:47 pm)