How long should a news post for the website or the magazine be?
A typical post with an image is about 300 words +/- 50 such as the examples shown in the second and third attachments below.
Once a post creeps past 400 words it gets cramped on a 1/2 spread and would perhaps need 2/3 of a spread and could be paired with a 200-word post to complete a spread - assuming there are no ad placements to consider.
On a typical spread, without any ads, there is room for about 600 words and images.
Long or short?
So how to decide whether a subject warrants 200 or 400 words, or more such as the full spread?
It is impossible to so set a general rule. Some subject matters are more interesting than others. It depends on the substance, how 'meaty' the story is and to what degree the subject is catchy.
Sometimes it there is just a little core of substance wrapped in a lot of circumstantial bla-bla-bla which is excess fat that needs to be trimmed off. Some of the leads are just like that. Some sources just take forever to get to the point and often the essence or interesting points aren't really highlighted but just blended in.
The art of editing
The challenge is to identify what is really significant and tease out the core issue so it stands clear. We need to get to the point in the shortest possible manner without losing essential info in the process, and by essential I mean essential and not an extensive backgrounder. It is not about including as much as possible. Au contraire, it is about trimming off everything that isn't essential.
Get to the point
In general, the shorter is better because we all have a tendency to get wordy or feel an urge to argue a case filling in too many details. But our news are briefings, not full backgrounders. They need to be concise, and get straight to the matter at hand and point out what the significance is and why.
Less is more
I have developed a technique during which I keep cutting the text still shorter by removing extraneous passages and filler words "until it breaks": I keep on distilling and reducing the text until it reaches a point where it cannot be reduced any further without losing something essential or the meaning changes. That is where you stop or dial it back one notch. It is not easy but you know when you are finally there. The resulting post will often stand out as being quite clear.
Remember the references!
News needs to be brief but also well documented. Readers who want more information should be provided with a straight pathway to further reading. Where possible reference the original source and not another news outlet.