In order to resize a widget, long-press on its name until a menu appears, then tap Resize.
Resizing is done by dragging one of the four sides to make it bigger or smaller.
Tap the name of the widget to bring up the menu for it, then select Edit widget to go to the Create/Edit Widget page.
At the bottom of the page, select either Slim, Medium, or Wide. The height of a widget cannot be adjusted.