Word count matters.
There is nothing that says, "I have no idea what I'm doing," faster or more clearly than a manuscript that is way out of the ballpark in terms of length.
However brilliant your concept or writing, if your manuscript is unusually long (or short) for its category, you're introducing obstacles to it being published.
If you want to give your submission the best chance, take a look at what else is being written in your area. It's common sense to understand the conventions and take on board what seems to be acceptable. Your picture book may be phenomenal, but at 450,000 words it's not going to be taken seriously - especially if you tweet it in 140 character instalments to your chosen agent.
Here, to help you out, is a great post by literary agent Jennifer Laughran. It covers the issue of word count in children's and young adult fiction and offers a helpful guide to the norms. There's even a useful link which allows you check the word count of other books like yours.
Because you want to maximise your chances - don't you?
JENNIFER LAUGHRAN'S GUIDE TO WORD COUNT