To make Text Bigger in Squarespace, you can follow these.
#1. Highlight text
#2. Click the Scale icon
#3. Result
#4. In case you want to change a specific word text to bigger only. You can highlight text and then press Ctrl U
Result
Then use this code to Custom CSS box:
div#block-d9fbcf76b56f4983068b p span[style="text-decoration:underline"] { text-decoration: none !important; font-size: 30px !important; }
Result
Note: div#block-d9fbcf76b56f4983068b is Text Block ID. You can use this tool to find the ID