One of the first questions I ask before I write for another blog is “What's the maximum width an image can be?”. It's an important question. If an image is too wide it will probably break the design. The height of an image is irrelevant in this respect but some blog owners don't want images to be too long vertically either.
I've tended to use 550 pixels on my blogs over the last few years. Most of the blogs I have written set the maximum width of their images at between 500 and 600 pixels. There are many blogs that exceed this width. For example, the technology blog Engadget uses a width of 620 pixels and The Verge use full screen images over 1,000 pixels in their reviews.
Many other blogs choose to use small images despite their designs being capable of displaying images much wider. Smashing Magazine tends to show images of either 500 or 550 pixels though their current design would easily accommodate 600 pixels.
The concern is what your website design will be like in the future. You may be using a large one column design just now that easily fits in huge images but who's to say you won't add an additional column in the future.
This was an issue I had to address when I recently relaunched this blog. I had been using 500 and 550 pixels before for blog post images however the current design is much wider and can fit images of up to 700 images. My concern was that by using wide images too wide, I am going to restrict what designs I can use in the future (unless I want to do a huge amount of editing).
In the end I decided to use a width of 650 pixels. My reasoning being that smaller images don't look great in wide content areas and in the future, I'll probably use a design that accommodates large images anyway. I don't plan overwhelming visitors here with advertisements so a second sidebar is not a concern. Mobile usage isn't something that worried me as most good mobile designs reduce the width of an image to fit the screen better.
What is the maximum width you use for your blog posts? I'd love to know whether you are using the maximum width your current design supports or if you are using shorter images so you can choose other designs in the future.