There are two modes in the widget:
• Where you fix the widget height by number of Tweets, in which case the widget size is variable depending on the content of those Tweets, but remains static after page load.
Or,
• Where you fix the pixel height of the widget, which remains that size in the page regardless of content, and viewers can scroll through Tweets and load more.
Setting the height as number of Tweets overrides setting the height by pixels. There is no mode to set the height in pixels and cap the number of Tweets that can be scrolled through (since more Tweets will load, and more Tweets can be loaded by the viewer dynamically.)
Ben