Select the item you'd like to resize.

A box will appear around the item, with bars in the center of all four sides and dots on each corner.

Click and drag the dots to enlarge or shrink the item, while maintaining the original proportion. 

Clicking and dragging the bars in the center of the sides will resize the item, as well, but it will alter the proportion of the item.

When you're finished resizing the item, click on any blank area of the canvas or press the esc button on your keyboard to deselect the item.