According to the answer to this question: Plurals (from The Chicago Manual of Style Online), an acronym should be treated as a word with its own meaning, so (extrapolating) the "s" indicating plural should have the size of the lower-case "s" corresponding to the font size of the acronym. 

Invoking the smaller option for the acronym package results in typesetting the acronym using \textsmaller from the relsize package. The "s" indicating the plural form will have the size of the lower-case "s" with \textsmaller, so I think that the right option would be to leave the size of the "s" as it is. Furthermore, increasing the size for the "s" could result in confusion if the size of the "s" is too close to the size of the singular form of the acronym.

However, if you really want to change the size of the "s" indicating plural forms , you need to redefine the internal commands \AC@aclp, and \AC@acsp. The simplest way to achieve this is to patch those commands through the etoolbox package. A little example, in which the "s" will have size given by \normalsize:

\documentclass{article}
\usepackage{etoolbox}
\usepackage[smaller]{acronym}

\makeatletter
\patchcmd{\AC@aclp}{\AC@acl{#1}s}{\AC@acl{#1}{\normalsize s}}{}{}
\patchcmd{\AC@acsp}{\AC@acs{#1}s}{\AC@acs{#1}{\normalsize s}}{}{}
\makeatother

\acrodef{DLL}{Dynamically Linked Library}

\begin{document}
The plural of \ac{DLL} is \acp{DLL}.
\end{document}




When using the acronym package and the smaller option, how to avoid the tiny 's' in the plural acronym?
The state of California is expected to be hit by a large storm in the coming days and there are frequently clogged drains in my town. I have an electric vehicle that seemed to handle driving through large puddles during the last storm but I was curious if anyone could tell me what the potential hazards are of driving an EV through a large standing pool of water. 

I don't need to worry about getting water in the engine (I think) since there is no air intake. There are fans that cool the battery if it gets too hot but I don't think that will be a problem during this storm.

EDIT: I have a Ford Focus
