I get mine from the thumb. For example
right click the thumb and select properties. It will be something like this
http://farm3.static.flickr.com/2081/...705dbdb9_s.jpg
From here you check and see what's available. Look at the end of the address _s.jpg by changing this you can get different sizes.
change the s to a b for the largest size but that may not be accessible. for example
http://farm3.static.flickr.com/2081/...705dbdb9_b.jpg
If you get unavailable remove the _s and it will look like this
http://farm3.static.flickr.com/2081/...67705dbdb9.jpg
I hope this helps