A popup should have a minimum and maximum width defined to prevent it from being presented as either too narrow or too wide. Maximum width should not exceed 50% of the viewport.
There should be calculated space enough in top and bottom to account for the height of the close button (X).
If for some reason the content in a popup is so excessive that it can not be fitted into a modal of maximum height and width, then the content in the popup should scroll. Not the actual pop-up.
The popup should have a descriptive header for the ation/message in question. Read design guidelines for labels on buttons for info regarding this.