You can use the snap grid to assist with sizing and positioning shapes.
The default settings are Snap to Grid on, and Show Grid off - you can switch either of these on or off (in any combination) from the toolbar, the View menu, or via the keyboard shortcuts shown on the View menu.
With the snap grid turned on, when you draw or position a shape, the shape's nodes snap to the nearest grid point, which provides an easy way of achieving exact sizing or positioning. Snap to page margins works in the same way.
NOTE: Nodes and lines snap directly on top of grid lines. Options to snap inside or outside grid lines may be added in the future.
You can change the grid spacings via the Document Inspector - grid settings are applied document wide.
Other sizing and positioning resources include the mouse crosshair (turn this on/off from the toolbar, View menu, or keyboard shortcut), the Shape Inspector and shape nudging by using the keyboard arrow keys.