Adjust height for MAINCONTAINER

Hi all,
i have built my website on wobbly.com. i iused one of thier templates which i adjusted using the css.
i am stuck on trying to make the heioght of the main container shorter. it is too long.
in the css i tried to add a height; 400px; and height; 50%; line but nothing happens.

here is what i see

.container,
.main-container {
width: 770px;
margin: 0 auto;
}

.main-container {
position: relative;
top: -50px;

any ideas on how i can do this?

thanks in advance !