Really what you care about is the ratio of wingspan to body length in the kind of moth you’re creating. There are a lot of different kind of moths out there. Find the one you like, then math it out.

• Find your ideal moth
• Google that moth’s wingspan, “A”
• Use a tool like maybe pixels in a line or even a ruler against your computer screen to find the ratio. You want wingspan, “W” and height “H”
• The ratios of real world measurements to what shows up on your screen will be roughly the same, so A/W = X/H. You can find the approximate real world height with X = H * A / W

Or just look at the moth and make a best guess. Anyone who calls you out can do the math themselves ;p