You shuld be able to click on the border and drag it to a bigger size

Assuming you mean live chess the window sizes can often be really troublesome.  RedSoxpawn is right that clicking on the border (the bottom of the page in this instance) is what you want to do, but it's tough to actually get it to expand in live chess if you're unable to drag far enough down.

If you're having trouble, I often drag the window upwards on the page first so that I can drag the bottom border far enough down without my mouse hitting the bottom of the screen.  This usually works.