photographer website please HELP

[jedphotos]

Hi, i'm a french photographer and i have a simple quesiton that i can't figure out.

I explain my project: I would like to have different prices depending of the size of the picture.
for example:
9X13 = 5$
10x15=10$
ect.....

doest some one have an idea how to set up this ?

thx for taking time
sincerely

[John2400]

http://extensions.virtuemart.net/products/custom-size-detail