I look small, and flat in the morning simply due to being dehydrated.
Half a gallon down with sea salt, and it's a completely different look. I'm a 2+ gallon of water a day person; especially in the summer.
There is no rule of thumb here. There are no biomarkers for hydration; maybe Stewie and the other smarties can chime in here. Sodium and minerals play a big role here as well.
Often times, if you've been under hydrating your body, that's the homeostatic set point for your body. Just like if you under consume sodium; your body releases it at a slower rate.
But that doesn't mean it's ideal. I'd gauge it off performance in the gym, and healthy blood work. Something you need to auto-regulate yourself.