Thanks for contacting us.
Let’s answer questions one by one:
1/ Try to disable all the third-party plugins (including cache plugins), that do not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins , clear cache of browser, then check. Maybe one of the additional plugins is the reason for the problem.
When I apply the 300×300 size – everything is good, the but images are blur. – do you mean images change depending on color selected when this size is applied? If so, try to regenerate thumbnails using plugin – https://wordpress.org/plugins/force-regenerate-thumbnails/
Let us know results.
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