I have seen this style of card in books and on the internet and have been meaning to get around to making one...of course when I wanted to do it - I couldn't remember what they are called and couldn't find one. So I sorted out the measurements myself. I still don't know what to call it! I must make a pdf file but hope that you can follow the diagram for the measurements, as they are easy to cut out, you can see from the image the shape you need to do, the centre bits need to be around 2.5 - 3cms/1 inch I reckon to be in reasonable proportions. This one closes to be a square card.
So you need to make sure that you cut the black lines and only score the orange lines - do not cut. Folding one set in the opposite direction to the other. You need to decorate the sentre of your card as you choose, I used Hero Arts stamps as they complemented. Follow the photo to see how it should look.
Don't forget you can make things bigger and smaller in the Craft Robo Software, so you should be able to keep the proportions.
GSD main file here
GSD copy for inner paper cut out
It is has been saved in version 5.1 - which I downloaded from the Silhouette site for free:
© Carolyn Woodruff February 2010