A full, printable Media Card can be found here (unfortunately this is NOT mobile-friendly) please only open this on a Flash-enabled computer - open Media Card
Answers to just about every advertising-related question can be found HERE (the info at this link IS mobile-friendly!)
A few key tips...
Size (in this case at least) doesn't matter! It's the number of ads you book, that gets the discount!
You won't be asked to pay anything when you book - it's pay as we publish - more info
You can get up to 38% off each add if you book 4 ads at the same time (or 15% if you book two ads, 28% if you book 3 ads)
A 'first time advertiser discount' is available if you haven't advertised with us before - more info