Question

It says to use a ‘-‘ but that doesn’t work for me..
Size Suffixes

The letter suffixes are as follows:

s   small square 75x75
t   thumbnail, 100 on longest side
m   small, 240 on longest side
-   medium, 500 on longest side
z   medium 640, 640 on longest side
b   large, 1024 on longest side*
o   original image, either a jpg, gif or png, depending on source format

http://farm6.static.flickr.com/5025/5680710399_b609135279_-.jpg

Answer 1

http://farm{farm-id}.static.flickr.com/{server-id}/{id}_{secret}_[mstzb].jpg

In the explanation, they are using - as a symbol for “no suffix at all”, so the URL

http://farm6.static.flickr.com/5025/5680710399_b609135279.jpg

is what you’re looking for if you want 500px length.