Although you can specify the page width and height using the page properties. These values aren't fixed.
When you move an object outside the current page size it will automatically adjust the settings to make everything fit.
You can however enable Guide Borders so you can make sure all objects stay inside certain limits.
Select Layout|Guide Settings and enable Show Guide Borders.
Related topics:
viewtopic.php?t=33
viewtopic.php?t=153