click on your photo, click on it again so you get the black screen with the picture then top right hand corner click view all sizes, then choose your size, then right click on the image and choose "Copy Image URL"
there's also a usefull feature for forums posting like here. click on your photo, above it you'll see a "share" menu, then BBCODE. select the size you want and cut/paste the code as is in your post.
the picture appears with your name and title and is a clickable link to your flickr.