I'm voting for 1.75inch. Tried to check the belt height by comparing Jude Law's height (1.82m) from the cutout against a screenshot from episode 7 where the belt is visible and he stands relatively straight. Green bar is approximately 1.82m, and I then compared this with the approximate belt height (red bar). Higher res pictures gave me lots of pixels to do some calculations, ended up somewhere around 4.8cm (~1.89 inches). Considering that this is only approximate (think a few pixel less for the red bar make a big difference), and this number is heigher than 1.75in, I feel quite comfortable with this guess.
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Also, here's the belt screenshot from this scene:
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I laughed so hard. I couldn't find one where he stood straight AND was showing the buckle. This is a great method with the split pic. I came up with the same rough measurement after using your split pic in my measurement app. Looks to have between 3.74 to 4 inch total width on the actual ornament buckle. The upper and lower belt look to be the same width as well but the upper is curling due to its position into the abdomen so presents slightly smaller visually.
I also found a really good full height pic at a company that does cardboard standups. I cannot vouch for them as a company but I am posting the link here.

Jod Na Nawood Cardboard Cutout
This is a cardboard cutout of Jod Na Nawood from Star Wars and Lucas/Disney's Skeleton Crew. Easy to set up, it features a single-sided, high-quality print on cardboard
After some allocation of where to start and stop total height, what his current height is (varies depending on info source) at 5 10 1/4 or 1.78m, the belts are comfortably within range to be 1.75 (as presented by Modery) and the buckle itself at 4 inches wide (variance possibly due to angle of 1/8 minus) by 2 5/8 inch (Much harder to cement this measurement). This also presents the option of using these as known sizes for further outfit measurements.
I am exactly his height and approximately 6 months older so test runs should be significantly easy to judge for necessary corrections.


