The Stampin' Up! card stock comes in 2 sizes 8 1/2x11 and 12x12 inches. The 8 1/2 x 11 is usually what I use for a basic card. It can be cut easily in half to create a basic card. It can be cut vertically to create two (2) 4 1/4 x 11 pieces or horizontally to create two (2) 5 1/2 x 8 1/2 pieces. Either of these folds in half to create a 4 1/4 x 5 1/2 card.
Once you have your card base then you want to add the layers! Some people like very thin mats on their layers and measure in 1/8ths of an inch. I very rarely do this as it feels more difficult and doesn't usually change the view/look a lot. I generally cut mats that are a 1/4 inch smaller than the previous layer which when centered leaves a very nice 1/8th inch all the way around. You can get four (4) 4" x 5 1/4" layers from one sheet of card stock. The card stock is cut 1/4" smaller in each direction.
If you choose to use foil paper or even card stock you can die cut or punch something from the center to use later if you plan to cover this layer with another layer of card stock. I do this frequently to save paper or to decrease the weight of the card.
The next layer is cut a quarter inch smaller than the previous layer or a half inch smaller than the card base (front). It will be 5" x 3 3/4". If you are planning to add some designer series paper to your card this is a great way to bring in 2 colors from the Designer Series Paper. You could add 4 pieces of Designer Series Paper or a strip down one side. These are easy layouts that work well for lots of card types/occasions.