extension joomla, template joomla,banner extension joomla,jomla slider,slider joomla

in the menu at the top go to 

site --> template manager --> site templates joomla

select the template you are using, and click the 'edit css' button.

You can find the font size in there.

You can change this::

note: mega is type of you fix in template

HTML Code:
ul.level0 li.mega a.mega {mega.css (line 36)
    border-left: 1px solid #666666;
    border-right: 1px solid #333333;
    color: #CCCCCC;
    display: block;
    font-weight: bold;
    line-height: normal;
    margin: 0;
    padding: 8px 15px;
    text-decoration: none;
}

to

HTML Code:
ul.level0 li.mega a.mega {mega.css (line 36)
    border-left: 1px solid #666666;
    border-right: 1px solid #333333;
    color: #CCCCCC;
    display: block;
    font-size: 16px;
    font-weight: bold;
    line-height: normal;
    margin: 0;
    padding: 8px 15px;
    text-decoration: none;
}

and adjust font-size value accordingly

2) Change this::

HTML Code:
ul.level0 {mega.css (line 21)
    border-right: 1px solid #666666;
    float: left;
    margin: 0;
    padding: 0;
}

to

HTML Code:
ul.level0 {mega.css (line 21)
    border-right: 1px solid #666666;
    float: none;
    margin: 0 auto;
    padding: 0;
    width: 400px;
}

and adjust width value in accordance with no. of menu items placed