Headings: Collection title elements

Change collection title size:

/* -- code to change collection title size -- */
.template-collection h1 {font-size: 1.1em;}
/* - end - */

You can adjust the height value to your needs. Change in increments example: change 1.1em; to: 1.2em; or 1.3em; or 1.4em; or 1.5em; etc

Make the collection title similar to the pages title:

/* -- code change collection title style to page style -- */
.template-collection h1.title {
  font-family: $titleFontStack;
  text-transform: uppercase;
  letter-spacing: 1px;
  -webkit-font-smoothing: antialiased;
  font-size:2em;
  color: $colorTextBody;
  font-weight: normal;
  margin: 0 0 0.75em;
  line-height: 1.4;
}
/* - end - */

To change collection description text size:

/* -- code to change collection description text size -- */
.content--flex {font-size: 14px;}
/* - end - */

Where do I add the code?

Open your Code Editor. 

From your list of themes, find the Actions link for the theme you'd like to edit. Then select Edit code. Example draft theme:

Example if you're working from your main live/published theme:

Quick link if the theme is live/published:
Open Code Editor

Next, in the Code Editor find your theme's CSS file, this will vary depending on which version of Pipeline you're running:
Which version of Pipeline am I using?

Once you know your version, find the right file in the left Assets folder:

Pipeline 4 - If you're working with a draft theme, open this file:

You'll be opening this file:

assets/theme.scss.liquid

Quick link if the theme is live/published:
assets/theme.scss.liquid

Earlier versions of Pipeline - If your theme is older than Pipeline 4 open this file for draft themes:

You'll be opening this file for earlier versions of Pipeline:

assets/style.scss.liquid

Quick link if the theme is live/published and an earlier version of Pipeline:
assets/style.scss.liquid

Add the code to the very bottom of the file on a new line. Make a couple blank lines before pasting the code snippet. 

Example:

Save the file after adding your code.