Not possible to resize images in blog posts and pages.
- In a Confluence site using the new editor for blog posts, create one
- Add an image
- Click the image
You would see buttons that would allow you to resize the image
There are buttons that allow you to change the image's display in reference to the text (Aligned to Left, Right, etc)
But there is not way to define the image's size through presets (Small, Medium, Large, Original) or manually inputting its size in pixels.