Are settings that best suit contents for a smaller paper size guaranteed not to produce underfull/overfull warnings for a larger paper size?

Scenario

I want to adopt the philosophy "typeset once, produce any size". It means that from the beginning I specify the minimum paper size (e.g., A6) and configure the contents to best suit the paper dimension. Once the development phase has completed, I can compile the input file to produce PDF outputs with any larger size (e.g., A5 and A4) only by changing the paperwidth and paperheight.

Assume that I am satisfied with the font and graphics dimensions so re-scaling is no longer needed for printing in any larger paper size.

This is my response for Yiannis Lazarides' comment: I want to produce A6 for iPod, A5 for iPad and A4 for printout/on-screen.

Question

Are settings that best suit contents for a smaller paper size guaranteed not to produce underfull/overfull warnings for a larger paper size?

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No, in general. With small line length you'll have to solve many overfull and underfull problems, that wouldn't appear with larger line length, which will pose new ones, instead. –  egreg Jun 14 '11 at 18:10
.. puzzled, why would you want all these different paper sizes? Normally one would have a screen version and another for paper. –  Yiannis Lazarides Jun 14 '11 at 18:52

In general, it is not possible as egreg mentioned. You can expand the code below, though to cater for various sizes and have all the corrections in one place. Here I used \ifpdf, but you can define your own conditionals \ifafour, \ifathree etc., in a similar fashion. Strange, you want A4 for screen I normally go smaller.

\documentclass[11pt]{book}
\usepackage[svgnames]{xcolor}
\usepackage{ifpdf}
\usepackage{lipsum}
\newfont{\HUGE}{cmr17 at 96pt}
\setlength{\textwidth}{3.0in}
\setlength{\textheight}{5.125in}
\setlength{\oddsidemargin}{0.0in}
\setlength{\evensidemargin}{-0.375in}
\setlength{\topmargin}{-0.375in}
\setlength{\footskip}{0.25in}
\setlength{\parindent}{0.5\parindent}
\ifpdf
\pdfpageheight 7.5in
\pdfpagewidth 4.625in
\else
\fi
\newcommand{\hstroke}{\rule[0.5ex]{5.0em}{0.2ex}}
\begin{document}
\newpage
\thispagestyle{empty}
\begin{center}
\setlength{\unitlength}{1.0em}%
\begin{picture}(45,16)(0,0)
\put(-0.625,0){\framebox(6.2,7.25){\HUGE \textcolor{purple}{A}}}
\raggedright FOR ALL\linebreak
\TeX nicians\linebreak
}}
\end{picture}
\hspace*{-0.6em}\rule{1.08\textwidth}{0.3ex}\\
{\small \sc by}\\
{\small YIANNIS LAZARIDES}\\
{\footnotesize Author of Gala Day Luncheons''}\\
\vspace*{18.5 ex}
\rule{\textwidth}{0.3ex}\\
{\small
BEIJING\phantom{ZZZZZ,}\hfill\raisebox{0.5ex}{$\bullet$}\hfill THE PANDA\linebreak
CHINA~~~~\hfill\raisebox{0.5ex}{$\bullet$}\hfill PRESS
}
\end{center}
\newpage
\thispagestyle{empty}
\vspace*{18ex}
\begin{center}
{\em Copyright, $\mit 2011$}\\[5pt]
{\sc By Xport \& Company}\\
\hstroke\\
\vspace*{18ex}
{\sc A little cook book}\\
%{\sc for a little girl}\\
\vspace*{12ex}
\end{center}
\newpage
\frontmatter
\thispagestyle{empty}
\vspace*{20ex}
\begin{center}
{\scriptsize FOR}\\
{\large \bf Knuth, Lamport and Carlisle}\\
\smallskip
{\footnotesize AND THE REST\\
OF THE CROWD THAT LOVES\\
TYPOGRAPHY''}
\end{center}
\newpage
\thispagestyle{empty}

\newpage

\mainmatter
\chapter{My Recipes}
\lipsum
\end{document}

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@xport Think about the future, me reading your work on a laptop at the beach! –  Yiannis Lazarides Jun 14 '11 at 19:43
@Yiannis: A6 and A5 are for the future. –  xport Jun 14 '11 at 19:51
@xport You can simplify quite a bit with the geometry package. I was just too lazy to look up the manual:) –  Yiannis Lazarides Jun 14 '11 at 19:54