You must be using the default home app to resize widgets.
Only the Motorola widgets can be resized.
Here is a Moto Calendar widget:
To resize long press on it and you will see:
Grab one of the 'knobs' on the corner and drag to resize.
When you are close to a size allowed you will see a green outline:
If you are near a size that is not allowed you will see a red outline:
If you release the widget with a green outline showing the widget will resize to that outline. If you release the widget with a red outline showing it will stay at it's current size.
Some sizes for the widgets may not display correctly in landscape.
Moto widgets are the only ones that can be resized using the default home app (unless you install a program/app that provides it's own resizing support).