I want to pipe a plaid design on a cake, but can't remember how to measure around the cake first. I know that it's an exact thing, or else the design will be uneven. Does anyone know how to do this?

I don't know. Didn't even know you needed to measure. Can't wait to hear the answer. I love plaid cakes.

Usually if I have to make something even I make a template using wax paper or parchment paper. Cut out a circle that is the circumference of the tier. Fold the circle in half, half again. Mark the points on the cake with a pin.
The stencils from Designer Stencils really dont require measuring.

is it a round cake? you can use this handy inexpensive wilton cake diving wheel http://www.wilton.com/store/site/product.cfm?id=3E30D66E-475A-BAC0-57C5D7D262D8EE2D&killnav=1

or if its square just use a ruler.

The cake dividing wheel from Wilton is probably the easiest and most exact way. It divides cakes up to 20" with up to 16 divisions. It comes in a kit with a great tool for stringwork/garlands, too! I love plaid-good luck!