Formatting different image sizes as featured image

I have got big NGO site to be done with lots of images but problem is mismatch of featured image sizes but there around 2gb images which are not of equal sizes ,which i need to add to featured image section

I tried dis https://slocumthemes.com/2014/05/adjust-set-size-featured-image-size-wordpress/ but no use ,also suddenly i cant c featured images on live site

so i need automatic solution that give the desired size for all the featured-image & slider http://test.uhfworld.org/journey-so-far-2/#past_project

I’t to make all big image in website to appear at same height & width in features image section like dis https://preview.themeforest.net/item/born-to-give-charity-crowdfunding-responsive-wordpress-theme/full_screen_preview/15709244

image should not stretched or blurr like dis http://www.uhfworld.org/ProblemStatement/