Opinions differ on whether sliders are a good idea from a customer perspective. Many sites these days tend to have just one core message rather than several rotating messages. But I think they can be useful if you have multiple audiences or a variety of products. From an SEO point of view it all comes down to file size. If you can keep the image sizes low, then fine. If they’re increasing your site load time then consider removing them. In terms of the correct file size for a slider, there isn’t one. Any slider will slow down your site and since they are not used, any files size dedicated to them can’t be justified.