I would like to replicate this chapter header design you see in the picture below. I guess titlesec is the way to go but I have trouble aligning the chapter name and the chapter number. Also, how do you handle very long chapter titles, so that they don't collide with the number?

• Welcome to the TeX.SE. It is possible to replicate it, but for my humble opinion it is an hard work to do. Aug 23, 2020 at 9:52

Here is an example of how it can be done:

\documentclass[12pt, a4paper,oneside]{book}
\usepackage{geometry}
\usepackage{newtxtext}
\usepackage[svgnames]{xcolor}
\usepackage{graphicx}
\usepackage[newlinetospace]{titlesec}

\titlespacing*{\chapter}{0pt}{*-4}{*8}
\titleformat{\chapter}[block]{\sffamily\bfseries\Huge\rlap{\hspace*{\textwidth} \scalebox{0.06}{\includegraphics{twd}}\vspace{-2ex}} \color{CornflowerBlue}\rlap{\rule{\paperwidth}{3pt}}}%
{\rlap{\makebox[\dimexpr\textwidth+\marginparsep+0.6 \marginparwidth][r]{\raisebox{-0.6\height}{\scalebox{2}{\thechapter}}}}}%
{0pt}{}%

\begin{document}

\setcounter{chapter}{3}
\chapter{Tweedledum\\[-1ex] and Tweedledee}

They were standing under a tree, each with an arm round the other’s neck, and Alice knew which was which in a moment, because one of them had ‘DUM’ embroidered on his collar, and the other ‘DEE.’ ‘I suppose they’ve each got “TWEEDLE” round at the back of the collar,’ she said to herself.

They stood so still that she quite forgot they were alive, and she was just looking round to see if the word “TWEEDLE” was written at the back of each collar, when she was startled by a voice coming from the one marked ‘DUM.’

‘If you think we’re wax-works,’ he said, ‘you ought to pay, you know. Wax-works weren’t made to be looked at for nothing, nohow!’

‘Contrariwise,’ added the one marked ‘DEE,’ ‘if you think we’re alive, you ought to speak.’

\end{document}