Size of YouTube Subscribe Circle and Video Rectangle
When using brand identifiers such as the YouTube Logo, you want to check what guidelines the brand has on using their brand resources. A quick Google search yields the official Brand Resources guideline by YouTube. Here you will find all you need:


  Minimum size
  
  The word "YouTube" should always be easy to read. So in digital
  content the Logo should never appear smaller than 24dp in height.
  
  The minimum size for applying the Logo in print is 0.125 in/3.1 mm in
  height.


Check the linked page for more details on the usage of the Logo. 

Sidenote: Since YouTube belongs to Google, they use the terminology of Google's Material Design guidelines. dp stands for Density-independent pixels, you will find an explanation on that on the Material Design website.