I need help with this one. Yes, change the font size for all PMs, Inbox, Sent, Archive.
I am just not able to determine exactly where in theme.css (CleanSlate) and forum.css (Resources) the default font size for PM messages in INBOX can be changed. sigh.
As noted earlier, this applies only to PM messages and NOT forum posts.
Currently there is no css class to change the font size.
You need to add a new Css Class to the ../pages/cp_message.ascx file
change the code from..
<td class="post" valign="top">
<%# this.Get<IFormatMessage>().FormatMessage(Eval("Body") as string, new MessageFlags(Eval("Flags"))) %>
</td>
to ...
<td class="post privateMessageContent" valign="top">
<%# this.Get<IFormatMessage>().FormatMessage(Eval("Body") as string, new MessageFlags(Eval("Flags"))) %>
</td>
then you can change the font size from the theme.css or forum.css
.privateMessageContent {font-size: whatever you like}