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Hi
I am having difficulty designing a mobile site using breakpoints as the screen content is not limited to the devices actual size. This means that whilst everything look ok the user can slide the page to the left which makes it all dissappear. My question is therefore how do I set up the design page so that the page width is limited to the smartphones resolution either horizontal or vertical depending on how the user is holding the phone?

Perhaps there is a recent video on how to do this?

Without seeing what you have done or not it difficult to say. But with what you say about scrolling left it says you have an object or objects outside of the viewport. See the following: There is a large is empty space at the bottom or right side of my page.
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If you can scroll, then there is most likely an object outside the viewport.

See also this related FAQ "There is a large is empty space at the bottom or right side of my page."
viewtopic.php?t=63817