found the tip helpful. regarding the size of image being uploaded while adding a new product. will try that and give feedback.

adding extra product media has no effect on the display size on webste product page. it just adds another image of the product only viewable when you open a product.

this explains best for future ref.

also try default image shouldn't be smaller than 512 and shouldn't be larger than 1024px.
image ideally should be web optimized, which means, shouldn't be heavier than few hundred kilo bytes...