could you please clarify if you aim to resize the main blog feed or the blog post page?
As for the main blog feed, it is not possible as for now.
As for the post page, you have to check if the post page settings are accessible on your site. If you can access it, you can resize the post page while being in the post page in the editor. To check it, please look for the 'Go to Post Page' at the bottom of the main Blog design settings: