If you wanna modify your theme, always try use the inspect element function of Mozilla or Opera browsers (other i dont use). Right click on an element of the theme, then Inspect element, last option.
1) You can change the title font size:
Go to main.css ; /themename/css/main.css file ; edit with notepad++ to be more easy. Go to line 63 ( HEADER Group and change there the font size of the title.
ex: h1.site-title { font-size:2em; line-height:1.3; margin-bottom:.25em; margin-top:0; }
2) Other trick, you can edit the padding:
.site-header { padding-top:1.250em; padding-bottom: 3em; text-align:center; }
.site-header hgroup { padding: 2.5em 0 1.875em 0; }
Reduce the values.
you can use the method @KiryGNC mentioned but don’t edit theme css files directly, because every time you update the theme your changes will be gone, instead override existing css values by putting your css code to Custom CSS box Appearance > Theme Options > Style tab.