The digital handbook for this cartridge says to set the width to 23.54 which will give you a finished card height of 7". However, if you use that measurement it won't fit on a 12x12" mat. I changed the width to 20" which gave me a finished height of 4.75".
Here's my completed card.
After I had assembled it I played with the Design Space file and worked out that I could in fact get it to fit on a 12x12 mat using the size in the handbook if I did a little slicing and rotating.
First you need to click the Ungroup button at the bottom of the Layers panel. That will allow you to work with each layer. If you don't ungroup, Slice won't be available.
I sliced all of the sets of the flowers in half by placing a square (click the Insert Shapes button) over one half of the set, selecting both the square and the flower layer and clicking Slice.
Delete the square and the sliced copy and then the two sides of that layer can be moved individually. Design Space will automatically place them closer together on the preview screen.
Repeat for the other flower layers.
For the base layer place an 11.5" square on the mat and move it over the base layer. You can select the square and click Arrange, Send to Back so that you can see the base layer.
Use the rotate handle on the base layer and turn it so it fits diagonally inside the square.
Select the base layer and the square and click Attach
Lastly, go to the Layers panel and click the eye to hide the square.
Now when you click Go, all the layers will fit on 12x12 mats. ☺
Here's the finished DS file if you want to see it. ☺