There are two ways to change the size of an element in the SiteBuilder editor; scaling and resizing. Scaling lets you change the size of an element while maintaining it's proportions (aspect ratio) and also affects all of the components of complex elements such as galleries and tabs panels. Resizing changes the size of the elements frame and will only maintain the element's proportions if the shift key is held down while re-sizing. Additionally resizing does not always affect the components of complex elements, depending on the settings chosen, such as having disabled Scale To Frame.
To re-size simply select the element you wish to adjust and drag the handles on the element frames until you have reached the desired size. Holding down the shift key while doing so will keep the element's proportions.