set this up?

Example - The standard is a 4X6 Card on Lustre paper. The minimum to order is 25 and after that in 5 card increments.

I set up to look like this with a drop down:

Select card Quantity (minimum of 25)
25 Single Sided Flat Cards
30 Single Sided Flat Cards ( +$5.70 )
35 Single Sided Flat Cards ( +$11.40 )
40 Single Sided Flat Cards ( +$17.10 )
45 Single Sided Flat Cards ( +$22.80 )
50 Single Sided Flat Cards ( +$28.50 )

After that - I have other options for paper and information for the customer to fill out.

What's my problem? I am not sure how I should go about adding different sizes options. Options for sizes are from 4X6 - 5X8, with anything over 4X6 is an increase in cost, not much until 5X7. At first I added another drop down menu for the customer to select the sizes, but that was before I put in the option for the customer to get more than 25 cards. So I am not sure how to keep it simple for the customer and still enable to get the correct cost to the customer while maintaining my markup.

Any idea's?