It's very cool that it can be modded at all, but since the mod is called "better drag" and not just "different drag" I want to leave some feedback on the idea here. Maybe a mod with this exact function could be called something like "simpler drag" or "realistic drag" to avoid confusion?
So, here is this thing - if only the width limits speed, all ships should be thin and long. But if only the mass limits speed, then all ships must be as wide as possible to fit more engines. It's kind of realistic, but equally not fun as you can just make you ship whatever wide to fit more stuff on it. How about a combined option to incentivize building more varied shapes, when both super wide and super long ships are slower? Incorporating both parameters would even make a natural distinction - smaller ships are intrinsically faster but more vulnerable, while bigger ones can fit more stuff and easier to defend. It will put a rather hard penalty on ultra large platforms, but depending on how you see it it's a fair deal - You should try to make you ships more compact instead of just spamming more engines and platform tiles. Can instead make a bigger fleet if you need.
Balancing this could be tricky though, I suppose. Definitely need some testing with different configurations. You said "round platforms are unaffected" but platform's mass scales with the square of size while its number of engines and so the thrust is only linear? If you've accounted for that and made bigger mass matter less that means a horizontally tilable platform will become faster and faster indefinitely... It's not ideal for sure! I suppose that the mass drag is linear then? And bigger square platforms are actually still slower than smaller ones? So I think getting rid of the width drag coefficient altogether is rather problematic... Maybe you might consider adding some adjustable values for both ingredients in the mod's settings for people to experiment with? Oh, and sorry for any grammar mistakes, I probably didn't proofread my English enough.