You can control the size of a gallery on a page by using a gallery tag.
Galleries have options for their display that are set using the menu "Administration", "Manage Galleries" or "My Account", "My Galleries" menu.
Galleries can be viewed by other members or the public via the menu "Organization", "Galleries". Your organization may have moved or renamed these menu items.
An administrator can also mark a gallery as a "Home page Gallery" via the "manage Galleries" page. This place the gallery on your Home page.
Galleries have default widths that are used for their display. These widths cannot be altered unless you use a "gallery tag".
When you edit a page you will notice a box that describes creating a gallery tag. It will generate a piece of text that you can place anywhere on your page. The gallery will then appear at the location. These tag have several useful options related to their display, including a width value. You can make the gallery larger on your page by specifying a larger width.