- alignment and reference point
You can adjust the alignment of a selected element via the Layout tab in the property panel.
 
Horizontal align:
There are three types for the horizontal alignment: Left, Center, Right, and left& right.
Whenever a type is selected there will be a reference position displayed in the “Distance” field. When you have defined a number in the “Distance” field, the element’s position will always keep this distance, no matter what the page size is. For instance, when you have defined the horizontal align to be “left” and the distance to be “200”, then the element’s position will be fixed to 200 pixel to the left of the page, no matter what is the page size is.
 
Vertical align:
Similar to the Horizontal align, vertical align can be set to top, middle, bottom and top&bottom.

Width and Height:
These two options stand for the width and height of the selected element. You can adjust the width and height by changing the corresponding values. The unit of width and height value is pixel by default, however you can change the unit to percentage by selecting "Percent" in the unit dropdown list.

- pixel vs. percentage
When the pixel is selected as the unit of the width/height of an element, the element size will be displayed in absolute pixels on your page and will not influenced by the size of its page.
While the percentage is selected as the size unit of an element, the element size will be vaiable depends on the size of its page, but the size propotion of the element on a page is fixed. The element size increases as the page size increases and vice versa.