Answer to: How do I push back on an impossible scope?

No-one else has pointed out that the senior staff may well know something you don't. This could be company politics, but it could also be development skills and knowledge. Once you've got sign-off that the list of must-haves and won't-haves (and anything in between) is carved in stone, make time estimates for the sub-projects. Ask the boss to review the list and see if they would find some stages of it to be a lot easier than you're thinking. He may be able to tell you about a new tool to use, or a new C++ class or library to use, or some such. They may also be able to clarify whether they're simply expecting a lot of overtime on it, which is taken for granted in many environments. I've worked huge amounts of unpaid overtime and learned a lot while doing so, in some cases, and in others had my approval rating skyrocket. I don't regret a second of it even looking back from my late 50s. Some seem to be cautioning to keep your concerns to yourself, but that's the last kind of coworker, team member or underlying I'd personally want. My career has always involved people from previous jobs and you want to be seen by them to have done the reasonable and prudent thing, and they'll want to rope you into the next company they work for. Just sitting and smiling while Rome burns isn't a trait that people will recall and try to hire you in a few years. On the other hand, it's always a battle to not be seen as a complainer or overly emotional either. I also would be careful not to give negative opinions of your boss or coworkers, etc., as that's not an attractive trait today's coworkers will be looking for the next time they're at a company that's hiring. To the extent you discuss your projects and work with coworkers (and I do all the time) it's up to you to spin it as: "the project seems to be pretty big, but the boss is pretty sure I'm being pessimistic, maybe I am, they probably know better than I do, let's see, I'll do my best."