CHANGE PARAGRAPH FONT SIZE AT SPECIFIC PAGE

Dear Sir,
I’m using HighEnd theme to build my website
Searching “paragraph font size” at the discussion page I found:
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Each page has it’s own class attached to the body element.
Simply use:
.page-id-999 p { font-size: 18px; line-height: 26px; }
In order to obtain the correct page-id-999 class, view source code of that page and look for body element.
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This could work for me me but I have two questions:
q1. Where/how to use (copy/paste)
.page-id-999 p { font-size: 18px; line-height: 26px; } ?
q2. Where/How I can view source code of page (so I’ll be able to look the page id ??

Thank you
Best regards
Haim