To export (save) edited images to use elsewhere, press the export button in the bottom left corner of the Lightroom window. This brings up a window that allows you to select the location to export to, the image size and resolution and sharpening.
If you are posting the images online around 2000px on the long edge at 72 pixels per inch at 85 quality, will give you a high quality image, of an acceptable size to upload. If you are exporting to print, the image should be exported at the appropriate size in inches or cm and at 300 pixels per inch (ppi) and at the highest quality.
