How to specify shape sizes using rmarkdown html notebooks

Question

I was not able to get rmarkdown html notebook to pay attention to the sizes of the shapes that I specify. I tried using knitr::opts_chunk$set(fig.width=7)and I tried to include the following:

output:
  html_notebook:
    fig_height: 5
    fig_width: 7
    toc: yes
    toc_float:
      collapsed: yes

What is the correct way to specify sizes and how can I resize an image for different pieces of code?

I got confused in the html-lab workflow as a whole, for example, to run all the code and then execute it, I used Run ... Run All and then clicking Preview. Is this the preferred method?

I use the latest packages rmarkdownand knitr, R and RStudio.