Making a fake figure that holds a text box of specified dimensions

A simple way to do this is to use \fbox{Dummy} with a \resizebox{<width>}{<height>}{<content>}. Maybe playcing the \fbox on the outside.

It is even easier and more flexible with my adjustbox package, e.g.:

\adjustbox{margin=1em,width=.9\textwidth,set height=10cm,frame,center}{Dummy}

---

\documentclass{article}
\usepackage{adjustbox}
\usepackage{lipsum}

\begin{document}

\lipsum[1]

\begin{figure}[!t]
    \centering
    \adjustbox{margin=1em,width=\textwidth,set height=4cm,set depth=4cm,frame,center}{Dummy}
    \caption{Dummy}
    \label{fig:dummy}
\end{figure}


\lipsum[2-3]

\end{document}

Another way could be using a \fbox to frame a \parbox (using the second optional argument you can control the height):

\documentclass{article}

\newcommand\FramedBox[3]{%
  \setlength\fboxsep{0pt}
  \fbox{\parbox[t][#1][c]{#2}{\centering\huge #3}}}

\begin{document}

\FramedBox{4cm}{4cm}{Some text}

\FramedBox{2cm}{8cm}{Some text}

\end{document}

Here's a way: an \fbox containing invisible rules as high and wide as specified

\newcommand{\fakebox}[2]{% #1 = width, #2 = height
  {\fboxsep=-\fboxrule\fbox{\rule{0pt}{#2}\rule{#1}{0pt}}}}

\fakebox{3cm}{2cm}

The initial setting will add no additional width or height due to the borders.

Adding a word is not difficult: I would never use a large word, just a small one in a corner to make clear that this is only a placeholder

\newcommand{\fakebox}[2]{% #1 = width, #2 = height
  {\fboxsep=-\fboxrule\fbox{\makebox[0pt][r]{\kern2\fboxrule\tiny dummy}%
     \rule{0pt}{#2}\rule{#1}{0pt}}}}