My say will be that why don't you consult to any professional web developer for this.

I usually use size 696px for width (< 100 KB) to optimize image before uploading on my website.

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I would say there are tons of plugins out there to optimize your blog images for better loading speed. One of the most important plugins you should check out in this regard is Smush It. It's a super powerful Wordpress plugin that will help decreasing images size without affecting the quality.

Furthermore, Have you checked JetPack plugin by Wordpress? It's having another important Image CDN tool which is called Photon. My advice here is to make sure to activate it as well, it will decrease your site pages loading speed significantly!

Also, There is another plugin called "Lazy Load". Although it has nothing to do with images sizes, it's mission is to turn on lazy loading on images, so basically images will load only when you scroll down onto the page. This way you're deceasing page loading speed smartly.

Let me know if you need further assistance,
Always here to help.

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