When you say same size are you talking about the dimensions of the image or size as in megabytes and kilobytes ?
EXport them all at the same size.
if you want a more helpful answer, provide more background info to your question. eg What type of web site?
I’m using Weebly to build my e-commerce website. I have multiple categories and multiple items within those categories. Can you suggest a proper size for the image? Thanks for your help
Don't Weebly say what size they need? If not, just export at a size like 2048 pixels and upload those. Weebly probably resize as part of the upload process.