OP, my 2nd is 3 in June (1st is 8) and my head on the whole "3rd" thing is about as messy and all over the place as this thread!
Firstly, as other have said, do not decide while still able to smell newborn on your little ones head. It is intoxicating and makes all sorts of sleep-deprived, fluids-covered, screaming, relationship wrecking, body ruining, hell-presence seem like rosy bliss!
Over the past almost 3 years I have had times when I have felt 100% at peace with having our 2 kids, looking forward to moving on, getting time for me again etc. Then a week later I am thinking about if we would need a double pram or if DS would be old enough to walk!
There are so many practical reasons to just stop now. It seems like the most sensible decision.
The biggest problem is though, our kids are just freaking awesome! My heart honestly aches for how much I love them, they make me smile every day regardless of how little sleep they allowed me to have the night before.
So many practical reasons though to stop at 2. I thought 40 would be my cut off, but here I am with a perfect cycle teasing me every month letting me know I'm still fertile! Then there's the thought of possible miscarriages again, another CS birth, Gestational diabetes, low milk supply. Bigger car, kids would need to share a room, with one in school I already have the beginings of all those expenses - plus the time demands which can't work around your schedule. What if baby screams nonstop for 8 weeks like our first? Or doesn't sleep through for 2 years like our second? Then there's work and needing time off again and and and...
Sorry, I'm probably no help. I keep hearing about people just knowing they are done or not done, however I just can't seem to get that clarity.