This varies considerably and all individual stamps carry only a part since the width of it, according to Andrew Norris, is approximately 28 mm across by 15 mm deep. In most examples the top bar is boldest and widest, the bottom bar being noticeably shorter.  However, the device is also seen inverted so that the shortest bar is at the top, and also rotated through 90 degrees.  In the normal orientation (see the image to the right), the lower bars appear to be closer together than the upper ones and some curvature of all bars is apparent.

See: Norris, Andrew, Bangkok, the British Post Office in 1882, published in Gibbons Stamp Monthly, February 2008.