Ivy here from Shopify.
The maximum resolution that be uploaded to Shopify is images of up to 4472 x 4472 pixels with a file size of up to 20 MB, however the optimum size for images varies depending on what you are trying to achieve. Could you tell me what you are hoping to do with the image? Is it a logo? or product images? Generally speaking, the best size to choose is 2048 x 2048 pixels for images, and it is usually best to keep them in a square format for product images. We have a helpful guide here on image resizing using the Shopify Image Editor which may be useful for you.
I am more than happy to give more specific advice once I have details on what the image(s) are going to be used for - I hope this helps and I look forward to hearing back from you!