How to change header notes font size?

Question

%----------------------------------------------------------------------------------------
%   HEADERS AND FOOTERS
%----------------------------------------------------------------------------------------

\RequirePackage[markcase=used]{scrlayer-scrpage}
\providepairofpagestyles{thesisSimple}{%
    \clearpairofpagestyles%
    \automark[chapter]{chapter}
    \ihead{\headmark}% Inner header
    \ohead[\pagemark]{\pagemark}% Outer header
}
\ifoot{}% Inner footer
\ofoot{}% Outer footer
\pagestyle{thesisSimple}
\providepairofpagestyles[thesisSimple]{thesis}{%
    \automark*[section]{}%
}
\providepairofpagestyles[thesisSimple]{review}{%
    \ofoot[\shorttitle/\authorname]{\shorttitle/\authorname}
    \ifoot[\today]{\today}
}
\pagestyle{thesis}
\ifbool{headsepline}{\KOMAoption{headsepline}{true}}{}
\PreventPackageFromLoading[\ClassError{\classname}{Package `fancyhdr' is
incompatible\MessageBreak with this class}{The pagesyles are defined 
    using package `scrlayer-scrpage', please consult the\MessageBreak 
KOMA-script documentation for details.}]{fancyhdr}


\NewDocumentCommand{\blank@p@gestyle}{}{empty}
\NewDocumentCommand{\blankpagestyle}{ m }{%
    \ClassWarning{\classname}{\string\blankpagestyle\space is
    obsolete,\MessageBreak use \string\setblankpagestyle \space  instead}\DeclareDocumentCommand{\blank@p@gestyle}{}{#1}
}
\NewDocumentCommand{\setblankpagestyle}{ m }{\DeclareDocumentCommand{\blank@p@gestyle}{}{#1}}

\DeclareDocumentCommand\cleardoublepage{}{\clearpage\if@twoside \ifodd\c@page\else
    \hbox{}
    \thispagestyle{\blank@p@gestyle}
    \newpage
    \if@twocolumn\hbox{}\newpage\fi\fi\fi%
}

Answer 1

With a layer pagestyle defined by scrlayer-scrpage the fontsize of the page header can be changed using \addtokomafont{pagehead}{...} or \setkomafont{pagehead}{...}.

Example with the standard book class (because there is no MWE in the question):

\documentclass{book}
\usepackage[markcase=used]{scrlayer-scrpage}
\providepairofpagestyles{thesisSimple}{%
    \clearpairofpagestyles%
    \automark[chapter]{chapter}
    \ihead{\headmark}% Inner header
    \ohead[\pagemark]{\pagemark}% Outer header
}
\ifoot{}% Inner footer
\ofoot{}% Outer footer
\pagestyle{thesisSimple}
\providepairofpagestyles[thesisSimple]{thesis}{%
    \automark*[section]{}%
}
\providepairofpagestyles[thesisSimple]{review}{%
    \ofoot[\shorttitle/\authorname]{\shorttitle/\authorname}
    \ifoot[\today]{\today}
}
\pagestyle{thesis}

\addtokomafont{pagehead}{\Large}% <- change the fontsize of header entries
\usepackage{blindtext}
\begin{document}
\Blinddocument
\end{document}