Change size of section, subsection, subsubsection, paragraph and subparagraph title

Standard classes

Here's an example using titlesec:

\documentclass{article}
\usepackage{titlesec}

\titleformat*{\section}{\LARGE\bfseries}
\titleformat*{\subsection}{\Large\bfseries}
\titleformat*{\subsubsection}{\large\bfseries}
\titleformat*{\paragraph}{\large\bfseries}
\titleformat*{\subparagraph}{\large\bfseries}

\begin{document}

\section{Test section}
\subsection{Test section}
\subsubsection{Test section}
\paragraph{Test section}
\subparagraph{Test section}

\end{document}

I used the simplified version of \titleformat (i.e., \titleformat*) since the only desired change was in size, but you can use the extended version if "bigger" changes are required. These are the default definitions for the standard classes (taken from the documentation):

\titleformat{\section}
{\normalfont\Large\bfseries}{\thesection}{1em}{}
\titleformat{\subsection}
{\normalfont\large\bfseries}{\thesubsection}{1em}{}
\titleformat{\subsubsection}
{\normalfont\normalsize\bfseries}{\thesubsubsection}{1em}{}
\titleformat{\paragraph}[runin]
{\normalfont\normalsize\bfseries}{\theparagraph}{1em}{}
\titleformat{\subparagraph}[runin]
{\normalfont\normalsize\bfseries}{\thesubparagraph}{1em}{}

so you can make bigger changes. The following image shows both the standard sizes and the ones obtained with the above modifications:

And here's an exampe using the sectsty:

\documentclass{article}
\usepackage{sectsty}

\sectionfont{\LARGE}
\subsectionfont{\Large}
\subsubsectionfont{\large}
\paragraphfont{\large}

\begin{document}

\section{Test section}
\subsection{Test section}
\subsubsection{Test section}
\paragraph{Test section}
\subparagraph{Test section}

\end{document}

According to an edit to the original question, the OP wanted also a sans serif font:

\documentclass{article}
\usepackage{titlesec}

\titleformat*{\section}{\LARGE\bfseries\sffamily}
\titleformat*{\subsection}{\Large\bfseries\sffamily}
\titleformat*{\subsubsection}{\large\bfseries\sffamily}
\titleformat*{\paragraph}{\large\bfseries\sffamily}
\titleformat*{\subparagraph}{\large\bfseries\sffamily}

\begin{document}

\section{Test section}
\subsection{Test section}
\subsubsection{Test section}
\paragraph{Test section}
\subparagraph{Test section}

\end{document}

KOMA classes

The above approaches are to be used mostly with the default document classes (book, report, article); if the document class used is one of the classes of the KOMA-Script bundle, then it's not advisable to use titlesec (See Incompatibilities between KOMA-Script and titlesec) but to use the features provided by the KOMA classes:

\documentclass{scrartcl}

\setkomafont{section}{\LARGE}
\setkomafont{subsection}{\Large}
\setkomafont{subsubsection}{\large}
\setkomafont{paragraph}{\large}
\setkomafont{subparagraph}{\large}

\begin{document}

\section{Test section}
\subsection{Test section}
\subsubsection{Test section}
\paragraph{Test section}
\subparagraph{Test section}

\end{document}

memoir

For the memoir document class, the situation is analogous: is also not advisable to use titlesec (See About memoir and titlesec incompatibility) but to use the features provided by the class; in the case of the lower sectional units, the class provides the family of commands \setXheadstyle:

\documentclass[article]{memoir}

\setsecheadstyle{\LARGE\bfseries}
\setsubsecheadstyle{\Large\bfseries}
\setsubsubsecheadstyle{\large\bfseries}
\setparaheadstyle{\large\bfseries}
\setsubparaheadstyle{\large\bfseries}

\begin{document}

\section{Test section}
\subsection{Test section}
\subsubsection{Test section}
\paragraph{Test section}
\subparagraph{Test section}

\end{document}

titlesec package options

For simple cases, you can use \usepackage[<size>]{titlesec}, where <size> is big, medium, small, or tiny.

big

medium

small

tiny

Example code

\documentclass{article}

\usepackage[medium]{titlesec}

\begin{document}
\section{A section}
\subsection{A subsection}
\subsubsection{A subsubsection}
\end{document}

You can just modify the sectional commands with the appropriate font size. Here's a small example in the article document class:

\documentclass{article}
\begin{document}
\section{A section}
\subsection{A subsection}
\subsubsection{A subsubsection}

\makeatletter
\renewcommand\section{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\normalfont\LARGE\bfseries}}% from \Large
\renewcommand\subsection{\@startsection{subsection}{2}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\normalfont\Large\bfseries}}% from \large
\renewcommand\subsubsection{\@startsection{subsubsection}{3}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\normalfont\large\bfseries}}% from \normalsize
\makeatother

\section{A section}
\subsection{A subsection}
\subsubsection{A subsubsection}

\end{document}

For a different font, replace \normalfont with the appropriate definition. For example, using \sffamily will yield: