Post interior margin in twenty eleven theme

Well, first, it’s a pure CSS question.

Second, padding is what you want to use.

Just change padding: 0; to whatever you like.

I just used FireBug to hack a padding of 1em into your code, and it worked like a charm.

// Edit

However, it is not just .entry-content but also .entry-summary that you want to affect.

.format-link .entry-content,
.format-link .entry-summary {
    padding: 1em; // for example
    font-style: italic;
    background: #d7b5b5; //sfondo rosa
}

// Edit – again

It seems, you have to work on your style.css.

For singular posts, you already defined a different padding:

.singular .format-link .entry-content {
    padding: 1.625em 0 0;
}

You either have to adapt that as well, or just delete it.