I read all your #RuinRunners posts on Note.com. I think what you are exploring and trying to do with an open project is very cool. I am more of a character artist myself, and struggle with environment art. I understand a 128px base unit for the character, since that is a standard unit in pixel art. I understand the importance of standardizing. For my own personal project ideas, I have given up on other art as I couldn't make online assets work well together, at the scale and quality I wanted. Instead I started using my own metrics. I have started to make a 2.5D template similar to what you described as a "belt floor action", but using an old-school style called "Cabinet View Projection" used in what were called "beat em up" games. Due to the 3D nature of my template, my units and scaling are a bit different. Example: I use the Imperial unit system, and my grid is not square due to Cabinet View Projection with a slight down wards looking distortion (to try to make things look flat). The result is 0.5ft or 12.7cm is 16x12pixels. In my system, your original 128 pixels becomes only 162.56cm (5'4") on my tile set (see message image). I figured I'd at least share some examples to inspire you, and to see if you might adopt it to your own metrics. Otherwise, Shaia would need to be 142 pixels tall to fit on the existing example template at 180cm. At this time, I don't have any plans (nor time) to do any art.
Viewing post in Shaia Ruin Runners/Character comments
It's hard to say without seeing the actual game screen, but judging from the attached image, and this is purely my personal opinion, I think it falls within the margin of error.
In reality, perfectly matching the scale of the background and characters is difficult. Therefore, I believe a difference of about one character's head height could be absorbed as an error.