Beginning in E-Prime 2.0.10.x, Slide, ImageDisplay, TextDisplay, and MovieDisplay objects have a default Width and Height of 75%. This was implemented to reduce average drawing time, since most paradigms do not actively use 100% of the screen. Reducing drawing time is beneficial because it may prevent delays in loading images. This is especially important for large or complicated images. Each object's OnsetDelay value reflects any delay in loading, which could include time in loading an image from the disk. The OnsetDelay value is logged in the .edat2 file for each object that has the Standard or Time Audit Only logging property enabled in its Properties > Duration/Input tab. More information on reducing OnsetDelay values can be found here: INFO: Steps to reduce large OnsetDelay values 
To change the Width and Height properties for ImageDisplay, TextDisplay, and MovieDisplay, open the object's Property Pages and navigate to the Frame tab. For Slide, select the SlideState you want to change, then open its Property Pages.
Note that Slide sub-objects are positioned and sized relative to their parent SlideState. For example, if the parent SlideState is set to 75% Width and Height, and a child SlideText sub-object is set to 50%, the SlideText will be drawn at 50% of 75%.
In E-Prime 2.0 Professional, the ToolBox Defaults feature lets you set the default frame size for all objects to 100%. For more information, please see this article and how-to video: NEW FEATURE: Toolbox Defaults  and http://www.youtube.com/watch?v=aviXAxGfRZI&feature=youtu.be