Less is more, but sometimes you have to provide depth. Instructions, giving clarification... maybe they just want it in writing. Sometimes we have to stretch our tolerance for length.
Bottom Line on Top
I don't care if they're looking for several answers. Give them the bottom line at the top. Give them the TL;DR.
What's the TL;DR?
TL;DR means too long; didn't read.
Recognize that they'll likely open the message on their phone. They'll catch the first paragraph, and save the rest for later.
Make sure they get the most important things they need to know at the top. A good rule for what goes into this pseudo-summary?
Any requests
Highlight where you need clarification
Deadlines
Set the expectation of what's in the note
Address concerns, but save deeper explanation for later in the note.
Our advice with Longer emails?
1. Pick up the phone.
Regardless of if you need to send a longer email or not. Following it or preceding it with a call is a great way to show you care and manage expectations.
2. Use bullet points
They're extremely effective for visuals. No one wants to read a giant block of text. If you break your points into clean bullets, it's easier for people to follow.
3. Keep it clear
Don't allow length to let you fall into a trap of using complex sentence structures and words. You can write a book at a 5th grade reading level. Don't let your writing style slip.
Comb through your "long email" a few times before you hit send. You'll be surprised how often that "long email" falls under 250 words after a few editing rounds.