The default page size in a beamer presentation is 12.8cm x 9.6cm (a 4:3 aspect ratio) with a font size of 11pt. The page size is set using geometry using the following additional default settings:
\RequirePackage[%
papersize={\beamer@paperwidth,\beamer@paperheight},
hmargin=1cm,%
vmargin=0cm,%
head=0.5cm,% might be changed later
headsep=0pt,%
foot=0.5cm% might be changed later
]{geometry}% http://ctan.org/pkg/geometry
In the above, \beamer@paperwidth and \beamer@paperheight is set based on the required aspect ratio. Also as mentioned in comments above, the values of head and foot may change, depending on the requirements provided by the template/theme used. Using these default settings, you can create your own slides in the article document class using:

\documentclass[11pt]{article}
\usepackage[%
papersize={12.8cm,9.6cm},
hmargin=1cm,%
vmargin=0cm,%
head=0.5cm,% might be changed later
headsep=0pt,%
foot=0.5cm% might be changed later
]{geometry}% http://ctan.org/pkg/geometry
\begin{document}
\textbf{\Large Frame title}
Here is some text on a ``frame''.
\end{document}
The basic idea behind keeping a small page size is to minimize (or "lessen") the amount of content on each slide, since viewers are not supposed to read manuscripts on the each frame.
For different aspect ratios, here are the other beamer defaults, declared using the key aspectratio:
\DeclareOptionBeamer{aspectratio}[43]{%
\ifnum#1=1610% 16:10
\beamer@paperwidth 16.00cm%
\beamer@paperheight 10.00cm%
\else\ifnum#1=169% 16:9
\beamer@paperwidth 16.00cm%
\beamer@paperheight 9.00cm%
\else\ifnum#1=149% 14:9
\beamer@paperwidth 14.00cm%
\beamer@paperheight 9.00cm%
\else\ifnum#1=54% 5:4
\beamer@paperwidth 12.50cm%
\beamer@paperheight 10.00cm%
\else\ifnum#1=43% 4:3
\beamer@paperwidth 12.80cm%
\beamer@paperheight 9.60cm%
\else\ifnum#1=32% 3:2
\beamer@paperwidth 13.50cm%
\beamer@paperheight 9.00cm%
\fi\fi\fi\fi\fi\fi%
}