This is not so much a WordPress questions as it is a simple CSS question, but adding this rule to your theme’s CSS should make the button text the same size as the surrounding text.
.navbar .header-right a {
font-size: 11px;
}
If you also wanted to reduce the overall size of the box that surrounds the text, you could adjust the padding (currently set to padding: 4px 12px 4px; in the theme’s CSS file).
.navbar .header-right a {
font-size: 11px;
padding: 2px 7px; // 2px padding on top and bottom, 7px on right and left
}
Further Reading: CSS Box Model, Padding.