is this iFrame HTML iFrame, or is it the element created with Velo?
it would be different elements, so we may need extra clarification. elements size in iFrame can be changed with the following values: width=“Y” height=“X” ; where X and Y are values for numbers
along with this, you want it to expand on click, which won’t be possible most likely due to some Velo limitation, please check this thread on Velo forum: https://www.wix.com/velo/forum/coding-with-velo/resize-a-html-component