Drawing boxes of any size and any location

Question

How can I draw rectangles of any size and any placement on the current page?

    \PassOptionsToPackage{dvipsnames}{xcolor} 
    \documentclass[landscape]{article}
    \usepackage{tikz,graphicx}
    \usepackage{pgf}
    \usepgflibrary{decorations.shapes}
    \usetikzlibrary{shapes.geometric, decorations.shapes,decorations.markings, shapes, fit, arrows, positioning, trees, mindmap, calc}
    \begin{document}
    \begin{tikzpicture}[remember picture, overlay]

    \draw [line width=1mm,opacity=.25] (current page.center) rectangle (3cm);

    \end{tikzpicture}
    \end{document}

Answer 1

It's a bit hard to understand your question, so I have to make an informed guess. Here is an example in which multiple circles and boxes are drawn, relative to the actual size of the page, as you have it right now:

\PassOptionsToPackage{dvipsnames}{xcolor} 
\documentclass[landscape]{article}
\usepackage{tikz,graphicx}
\usepackage{pgf}
\usepgflibrary{decorations.shapes}
\usetikzlibrary{shapes.geometric, decorations.shapes,decorations.markings, shapes, fit, arrows, positioning, trees, mindmap, calc}
\begin{document}
\begin{tikzpicture}[remember picture, overlay]

\draw [line width=1mm,opacity=.25] (current page.center) circle (3cm);

\draw [line width=1mm,opacity=.25] ($(current page.center)+(3,0)$) circle (3cm);

\draw [line width=1mm,opacity=.5] ($(current page.center)+(5,1)$) rectangle ($(current page.center)+(7,3)$);

\end{tikzpicture}
\end{document}

The example uses TikZ coordinate calculations to use positioning relative to the page center. Is this what you want? If not, please clarify. Since you are using "remember picture", don't forget to compile twice!

Answer 2

With PSTricks.

Ellipses and Rectangles:

\documentclass{article}
\usepackage{pstricks}
\usepackage[a4paper,margin=3cm]{geometry}
\pagestyle{empty}
\SpecialCoor

\makeatletter
\psset
{
    xunit=\dimexpr\textwidth/10,
    yunit=\dimexpr\textheight/10,
    linewidth=2pt,
}
\begin{document}
\noindent
\begin{pspicture}[showgrid=top](10,10)
\psLoop{20}
{
    \psframe[linecolor=red](!rand 801 mod 100 div rand 801 mod 100 div)(!rand 801 mod 100 div 2 add rand 801 mod 100 div 2 add)
    \psellipse[linecolor=orange](!rand 601 mod 100 div 2 add rand 601 mod 100 div 2 add)(!rand 1001 mod 500 div rand 1001 mod 500 div)
}
\end{pspicture}
\end{document}

Circles and Rectangles:

% A4 paper size 21.0cm × 29.7cm
\documentclass[pstricks,border=2cm]{standalone}
\SpecialCoor
\makeatletter
\psset
{
    linewidth=2pt,
}
\begin{document}
\noindent
\begin{pspicture}[showgrid=top](17.0,25.7)
\psLoop{10}
{
    \psframe[linecolor=red](!rand 1501 mod 100 div rand 2371 mod 100 div)(!rand 1501 mod 100 div 2 add rand 2371 mod 100 div 2 add)
    \pscircle[linecolor=orange](!2 rand 1301 mod 100 div add 2 rand 2171 mod 100 div add){!rand 1001 mod 500 div}
}
\end{pspicture}
\end{document}

About rand

Let's consider rand 501 mod 100 div that produces a real number between 0.00 and 5.00 inclusive.