I've used the largest scallop circle die in a standard set of Nestabilities to form the pop up and the pop up sits 3cm out from the card. This size works well inside a 15cm square card. I used quite a thick card, but it would be better made with thin card or even acetate to reduce the bulk. It can be used in a tent fold or side opening card.
The tutorial shows the basic method I came up with to make the pop up, but it can be tweaked as needed, eg, using a fancy bracketed square instead of a scalloped circle, increasing the size of the base shape, altering the depth of the pop up shelf etc.
I hope you find the tutorial useful, and easy to follow...it's my first!