There are two things. What feels comfortable and what your energy need is. And for the change in meal size it’s important how many meals you had before, if you ate according to your actual goals or what.
But I don’t know why my old meal sizes would matter. Unless I have problems with eating much more at once… But if one isn’t ready to eat properly, the solution isn’t eating the usual size of meal even if it’s 4-500 kcal… Starvation isn’t good, we need our protein and other essential nutrients too… And I for one can’t undereat for long. I don’t want it either, most definitely and my body whole-heartedly agree.
So I don’t think there should be a strong correlation between old and new mealsizes, it makes zero sense to me especially that many of us don’t have the same mealsize when we eat multiple times a day… My first meal almost always the biggest. But we should eat according to our goals anyway except when it’s not comfortably doable… But than OMAD may not be our way yet, again. I am not a forceful one and I can’t eat X calories because I decided so. I eat as much as I feel right. Food choices and timing can help changing my calories but there are limits.
1200 kcal is such a tiny amount that I surely wouldn’t go below…
To me, it’s not so important how many times I eat. I have a daily… vague goal but I eat whatever I feel like and tweak my woe if it’s not right. OMAD is just a simple way to distribute my meals, it’s not particularly special, I should eat the same, just in one sitting. Maybe it’s different for others, fine, each to their own, but mine makes sense to me and I can’t get away with small OMAD meals, no matter if I have much or little to lose. And I can’t condone eating tiny meals anyway, it’s starvation… Yeah, I would think it may easily harm my metabolism. Starvation works that way as far as I know.