How to customize header with Rmarkdown?

Question

I'm currently working on a pdf document using Rmarkdown on RStudio but I'm very much confused about to how to customize my YAML section. This is my actual code:

---
title: "The very basics of R"
author: "Alejandro C."
date: "March, 2020"
output:
  pdf_document: 
    latex_engine: xelatex
    toc: true

mainfont: Times New Roman
fontsize: 12pt
header-includes:
  - \usepackage{titling}
  - \pretitle{\begin{flushleft}}
  - \posttitle{\end{flushleft}}  
  - \preauthor{\begin{flushleft}}
  - \postauthor{\end{flushleft}}  
  - \predate{\begin{flushleft}}
  - \postdate{\end{flushleft}}  

---

What I want to achieve is the following:

• Use a bigger fontsize in my title (for example, 14) and boldface it
• Use 12 fontsize for author and date
• Use 11 fontsize for my document (I don't know if this must be specified in the YAML section)

I'm new on this topic so I really don't know where to start besides the R Markdown Reference Guide. Any help and advice will be much appreciated.

Answer 1

You lost the format of each element of title forcing flushleft environments instead of redefining \maketitle, but some like ...

fontsize: 11pt
header-includes:
  - \usepackage{titling}
  - \pretitle{\begin{flushleft}\huge\bfseries}
  - \posttitle{\end{flushleft}}  
  - \preauthor{\begin{flushleft}\Large}
  - \postauthor{\end{flushleft}}  
  - \predate{\begin{flushleft}\large}
  - \postdate{\end{flushleft}}

... must be nearly to what you want. You can see what font sizes are exactly commands as \large or \Large here.