In my replies, I generally point out that I'm writing to a specific short word length -- 400 to 500 words -- because that seems to be the "sweet spot" for potential publication in the most newspapers.
Some newspapers will go as high as 800 or even a thousand words. I suspect that in the Internet Age, some of them are quietly doing away with length limits altogether since they no longer have to shoehorn everything into a certain number of column inches on real paper. But 400-500 words is long enough to not be a mere "letter to the editor" and short enough to appease editors who still ruthlessly enforce guidelines written back during the wax-backed paper and Zip-A-Tone manual layout days.
As I noted in my old book on writing op-eds:
If you submit your article to a newspaper as is, at 500 words versus the 400 specified in the newspaper’s guidelines, it may still be published. In those guidelines, you’ll likely have noted that the editor "reserves the right to edit for length."
You can cut it down to 400 words. Or the editor can cut it down to 400 words. Who do you trust to know your thoughts and priorities better? Yourself or that editor? Do you want the piece to reflect your priorities, or his?
Cut it until it bleeds. Someone is going to.
It's mostly just that simple. I can't think of a single issue for which it's possible to cram all the relevant facts and arguments into a 500-word op-ed.
You can pick a couple of facts and a couple of arguments, and work hard to make those arguments compelling to the reader, or you can offer that reader a laundry list that gives him no reason to change his mind about anything.