Widgets can be resized by dragging from one of the 4 corners or the sides of the widgets.
All resizable widgets have a clear box/outline indicator.
Holding down shift will maintain the aspect ratio of a widget while resizing it.
To get the size of your widget pixel perfect you can use the resizing tool. simply chose the height and width that you desire.
Tip: Some larger widgets cannot be resized as they are fitted to the size of your page and the device type you are designing for.