How to remove "Chapter: [Number]" in the toc and on the page itself from particular chapters?

Question

I was given the following code for creating a "chapter" on top of the page, followed by the "title" of the page, which is displayed below "chapter".

This is the document class I use:

\documentclass[chapterprefix]{scrreprt}

I have the following problem: I would like only particular \chapters to display "chapter: [number]" in the toc before a "title". The title should still be displayed just without "chapter: [number]". Right now all my \chapters display a chapter: [number] in front of the title in the toc and on the page.

Also, how to I remove the colons that appear after "Chapter[:]" on the page (but leave them in the table of contents)?

Additionally, by applying the \documentclass that was given to me something changed. I don't know whether the font got smaller but my pages shrank from about 60 to 54. Any idea?

\pagestyle{headings} %chapter in toc and on page (numbering)

\renewcommand*{\chapterformat}{%
  \mbox{\chapappifchapterprefix{\nobreakspace}\thechapter\autodot:
    \IfUsePrefixLine{}{\enskip}}%
}
\renewcommand*{\chaptermarkformat}{\chapapp\ \thechapter\autodot:\enskip}
\RedeclareSectionCommand[
  tocentrynumberformat=\tocentrywithprefix{\chapapp},
  tocdynnumwidth
]{chapter}
\newcommand\tocentrywithprefix[2]{\mbox{#1~#2:}}
\renewcommand*{\raggedchapter}{\centering}

Answer 1

To remove the : from the chapter heading, remove the redefinition of \chapterformat. It was only redefined to add the colon.

Unfortunaly I do not understand what you really want to do.

If the prefix should be removed only in appendix, you can use

\documentclass[
  12pt,
  chapterprefix,
  %appendixprefix=false,% no prefixline for appendix chapters
  numbers=noenddot
]{scrreprt}
\usepackage{lipsum}% only for dummy text
\pagestyle{headings}

\renewcommand*{\raggedchapter}{\centering}
\renewcommand*{\chaptermarkformat}{\chapappifchapterprefix{\nobreakspace}\thechapter\autodot:\enskip}

\let\originaladdchaptertocentry\addchaptertocentry
\renewcommand*{\addchaptertocentry}[2]{%
  \IfArgIsEmpty{#1}
    {\originaladdchaptertocentry{#1}{#2}}
    {\originaladdchaptertocentry{\chapappifchapterprefix{\nobreakspace}#1\autodot}{#2}}%
}
\RedeclareSectionCommand[
  tocentrynumberformat={\def\autodot{:}},
  tocdynnumwidth
]{chapter}

\let\originalappendix\appendix
\renewcommand*{\appendix}{%
  \originalappendix
  %\renewcommand*\chapterformat{}% remove the chapter number  from chapter heading
  %\renewcommand*\chaptermarkformat{}% remove the chapter number from header entry
  \renewcommand*{\addchaptertocentry}[2]{% remove the chapter number from ToC entry
    \originaladdchaptertocentry{}{##2}%
  }%
}

\begin{document}
\tableofcontents
\chapter{Foo}
\lipsum
\appendix
\chapter{Bar}
\lipsum
\end{document}

Run three times to get

If you only want to use unnumbered chapters with ToC and header entry, replace \chapter by \addchap:

\documentclass[
  12pt,
  chapterprefix,
  numbers=noenddot
]{scrreprt}
\usepackage{lipsum}% only for dummy text
\pagestyle{headings}

\renewcommand*{\raggedchapter}{\centering}
\renewcommand*{\chaptermarkformat}{\chapappifchapterprefix{\nobreakspace}\thechapter\autodot:\enskip}

\let\originaladdchaptertocentry\addchaptertocentry
\renewcommand*{\addchaptertocentry}[2]{%
  \IfArgIsEmpty{#1}
    {\originaladdchaptertocentry{#1}{#2}}
    {\originaladdchaptertocentry{\chapappifchapterprefix{\nobreakspace}#1\autodot}{#2}}%
}
\RedeclareSectionCommand[
  tocentrynumberformat={\def\autodot{:}},
  tocdynnumwidth
]{chapter}

\begin{document}
\tableofcontents
\addchap{Introduction}% unnumbered chapter
\lipsum
\chapter{Foo}% numbered chapter
\lipsum
\appendix
\addchap{Bar}% unnumbered chapter
\lipsum
\chapter{Numbered}% numbered chapter
\end{document}