Regarding your issue of resizing the images, here are a few things for your consideration:
a) Shopify has a limit of 4472 x 4472 pixels for each image. So you cannot go beyond that.
b) For the aspect ratio, each theme has a separate recommendation for which aspect ratio will work best for what kind of image, whether it is your banner, collection or product image. Here you can find the image recommendations for Impulse, but you will have to play around those numbers a bit, and see which looks best.
I would recommend a 800 x 800 pixel image ratio minimum, as below that is too difficult to view on a laptop.
Hope you found what you were looking for.