After you have created a new post you can edit that post or other posts. To edit a post go into Post or All Posts.
Hover over the post you want to edit then select the Title of the post or Edit as shown in this image.
Change Media File Image Size
To change the size of a file to the dimensions as seen in Library in Media, select the image you want change, then select the Edit Image button as shown in this image.
This will open the “Edit Image” Window edit the size you want to make sure Advanced Settings is selected as shown in the image below. Here you will want to delete the CSS Class along with the width and Height (-300×140) at end of the Source as highlighted below.
Note: The width (300) and height (140) will match the numbers at the end of the Source (300×140).
Note: You will want to do this for all the images you want to resize.
Note: Select Update when finished editing.