I’m assuming you’re talking about the image size in the post before they are clicked on to view full size? If so you should resize them after you upload and insert into your post. This is done by clicking on the photo in the editor and using the handles to resize to your taste. This won’t affect the full size that is shown in the pop open lightbox when a user clicks on your image. You can upload as large as an image as you like however the size we use for competitions 1920 x 1200 should look ok for most except those with a very large monitor. Here is one @ 2560 x 1237 that I resized as described above.