Firstly, congratulations!! 
Secondly, a large family has always been a dream of mine, I grew up with 2 very older siblings so always felt like an only child and I'd rather have more than less! Lot's more fun!
Thirdly.. there are ways of giving your opinion.. be kind about it.
if they follow your lead, you’ll have 25 grandchildren. Depressing.
Bit late to worry now. I would think you are mad to have so many and careless and stupid to get pregnant again. Hope you make sure there is not a sixth
These aren't that kind are they? who are these to warn you off a sixth!
My sister has 2 kids, my brother will never have any.. so does that make it okay for me to have 3 or more? Yes there is an enviormental impact, but I am sure there is someone out there having their 6th or 7th so why should you feel guilty? it's happening now and there is no going back, so enjoy it. Are these people who are having enviromental concerns from your child changing their cars to fully electric? buying solar panels? not going abroad? not buying ANY plastic? no they probably are not. Good to some if you are but we aren't all saints.
A large family might be more green than a household of 2 people because you can teach them, unforunately adults nowadays not so much.
I would say just build a thicker skin because you are going to get people you know maybe thinking this and make sure you're prepared incase of a job loss etc, have a plan!