Length or height? Length is the width, or how wide it is. The footer area contains 3 widget areas which all together equal 1140px. You currently only have 1 widget active at the moment. Add content inside the other 2 if you want.
If you’re talking about height, that is determined by the padding on top and bottom and how much content you have inside your widget.
I checked your css and you have the exact same theme as the demo, you just need to add more content if you want a larger height, or you can increase the padding if you dont want to add more content. That can be done on line 2632 in your css, just remove the rem and use px. –
.footer-widgets .wrap {
padding: 90px 0 50px;
padding: 5.625rem 0 3.125rem;
}