What Anabel said!
Once you've started jumping around 2ft you should be jumping with your stirrups a hole shorter then you flat with. The higher you jump the more you'll need to shorten your stirrups (up to a certain point of course!). It's difficult to do proper flatwork with a short stirrup, it's difficult to jump well with a long stirrup. While it's not impossible, and you should be able to get by if needed, you're not setting yourself up for success. Although, while I do dressage exercises, I don't do them at quite the length of stirrup a true dressage rider would.