You can’t at the moment, because it’s auto generated.
But if you have no-XFBML, then you can try remove lines from 440 to 452 and replace it by $height = YOUR_HEIGHT;
.
Also if you have XFBML version, you can change it manually, but it’s a little harder. Go to line 426 and replace
$fblikebox = '<div class="fb-like-box" data-href="' . $url . '" data-width="' . $width . '"' . ($fb_like_settings['colorscheme']=="dark" ? ' data-colorscheme="dark"' : '') . ' data-show-faces="' . ($showfaces ? 'true' : 'false') . '" data-stream="' . ($stream ? 'true' : 'false') . '" data-header="' . ($header ? 'true' : 'false') . '"></div>';
by
$fblikebox = '<div class="fb-like-box" data-height="YOUR_HEIGHT" data-href="' . $url . '" data-width="' . $width . '"' . ($fb_like_settings['colorscheme']=="dark" ? ' data-colorscheme="dark"' : '') . ' data-show-faces="' . ($showfaces ? 'true' : 'false') . '" data-stream="' . ($stream ? 'true' : 'false') . '" data-header="' . ($header ? 'true' : 'false') . '"></div>';
I’m going to include possibility to simpler way in next release. Thanks you for your report. ??