The iFrame embed code can be easily customized directly from the embed code. Often when a form is embedded with an iFrame there is a default scrolling bar if the form is larger than the iFrame. In order to avoid this, there are two options that can be added to easily adjust the iFrame.
One option is to change the height and width of the iFrame to match the form by changing the numbers for the height and width in the code as shown below.
<iframe src="https://jk.formstack.com/forms/new_form_name" title="Basic" width="600" height="400"></iframe>
You can also remove the border and scrollbar. You can add scrolling = "no" and frameBorder = "0" to the embed code to remove this bar. Heads up: If you remove the scroll bar from the form, make sure to set the height so that the submit button does not get cut off.
<iframe src="https://jk.formstack.com/forms/new_form_name" title="Basic" width="600" height="400" frameBorder="0" scrolling="no"></iframe>
Note: The iFrame embed code is not mobile responsive.