I would rather do it slightly different using a \vphantom, it requires a little more manual work, but is easier to understand.
The reason why you get different size columns is the column specifier m and what it does. This particular specifier is actually a parbox. Obviously, since your text is longer than the width of the parbox it will overflow or get wrapped to the following line. As TeX's hyphenation patterns do not have a pattern for ooooooooooooo it just overflows out of the table, but this is another story.
In order to have all the cells the same hight, I would use a strut or TeX's equivalent a \vphantom in this case. A phantom command will create an invisible box with width zero but the height of the enclosing text. We do this with the command:
\def\Z{\vphantom{\parbox[c]{1cm}{\Huge Something Long}}}
This strut like in manual typesetting will make all the rows equal. The final code is as follows:
\documentclass[11pt]{article}
\usepackage{array}
\begin{document}
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}m{#1}}
\def\Z{\vphantom{\parbox[c]{1cm}{\Huge Something Long}}}
\bigskip
\begin{tabular}{| c | C{3cm} | C{4cm} |}
\hline
\Z Short Text & Short Text & Short Text \\ \hline
\Z Short Text & Short Text & Loooooooooooooooothetheme Text \\ \hline
\end{tabular}
\end{document}
If you have long repetitive tables of the same type, it might make it worthline to define a command addRow that can automate the addition of the strut.
By changing the parameter in the vphantom parbox to any of t,b,c, you can also get alignments at bottom, center or top.
As a sideline I changed Looo... to a word having an ending with a known hyphenation pattern and this time the overflow of text in the right margin also disappeared.
ArraystretchorextrarowheightorBigstrutswith the packagebigstrutbut in each case, some specifications are needed to get exactly what you want. – Alain Matthes Mar 06 '11 at 10:24\vfillbefore\centeringbut my answer is tricky. Perhaps there is something better with the use of package. – Alain Matthes Mar 06 '11 at 11:28fontencandinputencpackages and the\pagestyle). – TH. Mar 06 '11 at 12:02\mystrut– Alain Matthes Mar 06 '11 at 22:36p{#1}% <{\egroup}} -- is this a global command, will it affect every table in the document, or just a local thing?
– essay May 19 '14 at 11:23latexmk -pdfand resulted rows have different heights. – patryk.beza Sep 20 '17 at 17:12