To set the dimensions of an iframe when using the Static Html app, customize the height and width attributes. For example, the code below would have a width of 810 pixels and a height of 800 pixels.
<iframe src="example.com" width="810" height="800"></iframe>
Facebook tab dimensions are 810px wide and 800 px tall. It isn't possible to reduce the height of a Facebook tab to less than 800px.