If you are uploading your images to RC to post you will not have any control over the display size.
If you want more control, and also if you want people not members or not logged into the forum to see your images, the best thing to do is upload them to a dedicated image server like Photobucket, Flickr, etc. You can load images of different sizes there and link them back here on RC. Just use the Direct URL on the image server and copy it to the editor here on RC using the yellow mountain icon.
Be careful showing too large an image. Some people here are viewing via smart phone or low res monitor. Too big an image really blows out on these smaller screens.
Dave.M