Changing logo size and adding text beneath logo

Hi, I am a complete novice and found some code suggestions to put in the quick CSS to adjust the logo size but it is not working for me. I want to make the logo smaller than the standard 340px * 156px. I put in:
.logo img {
height: 100px;
width: 100px;
}

and it did nothing. I also tried:
/*header with social icons: */
.social_header #header_main .container, .social_header .main_menu ul:first-child > li a { height: 100px; line-height: 100px; }

and nothing happened. I would also like to be able to add some text directly beneath the logo.