[nerbert]

Scroll a little less than half way down template "search_input_post" and find and modify this code:

		<div class="blockrow">
				<label for="forumchoice">{vb:rawphrase search_in_forums}:</label>
				<ul class="multifield stacked group">
				<li>
					<select class="primary" id="forumchoice" style="width:500px;" name="forumchoice[]" multiple="multiple" tabindex="1" size="20">
						{vb:raw search_forum_options}
					</select>
				</li>
				<li>
					<label for="cb_childforums" class="rightcol">
						<input id="cb_childforums" type="checkbox"  value="1" name="childforums" tabindex="1" {vb:raw childforumschecked} />
							{vb:rawphrase also_search_in_child_forums}
					</label>
				</li>
				</ul>
		</div>

The size attribute determines the height