The Terra browser solves that problem. There is a full screen mode that hides the address bar and the tabs on the top. You'll notice that there is no address bar or tab bar on the top. The web page occupies the entire screen.
Now you can create an iPad prototype built using html, place it in a web site and show it inside a Terra browser to simulate the app.
The correct image size for the prototype is 1024 * 750. If you build an application with these dimensions, the prototype you create will display perfectly inside the Terra Browser while it is on full-screen mode.