In response to you response, I am a Karkat RPer and I see him as very short and slouched, so calling another character "tall" or "taller" from his perspective (without giving an idea as to how much taller) wouldn't be saying anything about the other character. (And because of his tendency to be melodramatic/to exaggerate, calling them a giant doesn't say much either.)
Sorry! I went out and just got back to this. Hope you're still around.
Okay, thanks for the clarification. That sort of thing is understandable and I'd probably let it pass. I'm usually angered by... I suppose an example would be good. I did a super hero prompt once and had someone pigeonhole my John as being short, thin, white, and (most annoyingly) bullied so that they could play the big strong savior and that is the sort of thing that bugs me.
I still think it's a good idea to run headcanons by your partner so you know where you're both (proverbially) standing, but there's a certain amount of slack each RPer will allow and I'm pretty sure your case falls within that territory.