It is always nice to make a card that is alittle bit different. When I want to do something, I end up fiddling about trying to decide on the right proportions. So I thought I would do a few projects, and put the measurements down that I used....then I can use them as a reference. 
H on the forum posted a challenge for this style card, and lots of us made  Christmas cards using her instructions.
So here are a few ways to cut your card depending what envelope you have and what card you have available:

Version 1

• Cut a piece of card - 11" x 4¼"
• Mark from the end 2" and then 3¾" make a score, then do the same working from the other end.
• Cut 2 pieces of fancy paper for the front flaps, 4 x 1¾ (stick on)
• Your back base will now be 3½" wide by 4" tall and each of the flaps 2" X 4" tall
  
  Version 2
  I wanted to make a few so changed the size so that they fitted envelopes I already had - C6.  So here are a few measurements and ideas for making one of these: so I cut an A4 piece of card to 15cm X 28cm - my card would end up 15cm high.
• cut a piece of card 15cm X 30cm
• Mark and score 5cm from the edge and then 9.5cm from the edge. Turn around and work from the other edge with the same two measurements
• You will now have a card with flap 15cm X 5cm and the back base 9cm X 15cm
  
  Version 3
  or for a slightly different card where the fold back flaps are the same size as the back - you can just stick two cards together and fold the flaps back, dead simple

You can see some of the entries and these will give you lots of ideas on how to finish your card  Here