Every website or online forum has it's own way of handling images. They often are not explicit - and may not know the optimum size. It can vary depending on where and how the image is posted. If images do not fit the required format, many sites automatically resize the image - and that may not be optimal. It may or may not include sharpening.
Generally smaller images are better than large ones. Some sites accept 1200 pixels on the width, but many others are closer to 800-900, or 1080 pixels. It usually depends on how much of the page shows the image. And each size needs to be optimized with sharpening. I compromise by using 900 pixels on the leng edge unless specified otherwise.
If you really need to show detail, post a 100% crop of the area in question.