There's a lot of good information on the jQuery UI stuff here.
You need to find where that height rule is coming from. Is it on the element directly as inline style or is it through a CSS class? If it's on the element directly, chances are there is a bit of JS running somewhere adding it. If it's through a CSS class, you just need to find that class and remove the height rule. If you're not hosting your own jquery UI CSS, you'll need to create an override rule in your style.css to override the height (set it to height: auto;). The override rule should use the same selector (set of classes and/or IDs used to target an element).
Just for future reference, this isn't really a development question; it's a skinning and design question so it's best to have stuff like this in that forum.