It gets stored as thumb/small/medium/large and original (if you have a pro account) - or something to that effect. If you don't have a pro account use the Flickr Uploadr program that resizes before hand to the large size (2068 x 1551 or something similar) - still smaller that most originals. Or even resize to 1600 x 1200. Saves a lot of bandwidth.
I have a pro account but resize everything to large (it's smaller than the original).
If you click on "all sizes" then a viewer can choose which size to see.