PenciDesign replied
Hi,
1 & 2/ You can use that image size. The theme will auto resize/crop it to make all your images display good on the site.
3/ When you insert images to your posts - I recommend you use larger size - check this image: http://prntscr.com/r3meii
And I recommend you use Classic block ( if you're using new editor from WordPress 5 ) to write your content:
Check this images:
Best Regards,
PenciDesign