htmltools::tags$iframe height code example

Example 1: set iframe height and width by js

#IFRAME_ID {
  width: 100%!important;
  height: 100%!important;
}
<iframe src="http://www.youraddress.com" id="IFRAME_ID"></iframe>

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If still not working you can try solution from on the solutions from the link
https://stackoverflow.com/questions/819416/adjust-width-and-height-of-iframe-to-fit-with-content-in-it

Example 2: set iframe height and width

#IFRAME_ID {
  width: 100%!important;
  height: 100%!important;
}
<iframe src="http://www.youraddress.com" id="IFRAME_ID"></iframe>