Different sizes of font available in table?

There's no \medium font size between \small and \large; you have \normalsize (which is the default font size). The standard font size switches are: \tiny, \scriptsize, \footnotesize, \small, \normalsize, \large, \Large, \LARGE, \huge, and \Huge.

By the way, you only need one font switch command inside the table and braces are not required since table forms a group, so you can simply say something like this:

\begin{table}
\small
\begin{tabular}{p{3.5cm}p{8cm}p{5cm}}
text & text & text
\end{tabular}
\end{table}

If you need to fine tune the size of a table, you can include it in a scaleboxfrom the graphicx- or graphics-package and scale the table to fit. If you need more options, you can use the adjustbox-package.

A warning: Arbitrary adjustments of different tables will result in different font sizes. This may look ugly.