Not the way I really wanted to do it, but this got me where I wanted to go. I added a div wrapper around the NeatHtml control within the Feed manager, and then I modified the rssmodule.css file to add the new class to it

<NeatHtml:UntrustedContent ID="UntrustedContent1" runat="server" TrustedImageUrlPattern='<%# allowedImageUrlRegexPattern %>'
ClientScriptUrl="~/ClientScript/NeatHtml.js" Visible='<%# ShowItemDetail %>'>
<div class="rssTextNeatHtml">
<%# RSSAggregatorUseExcerpt ? UIHelper.CreateExcerpt(DataBinder.Eval(Container, "DataItem.Description").ToString(), RSSAggregatorExcerptLength, RSSAggregatorExcerptSuffix) : DataBinder.Eval(Container, "DataItem.Description").ToString()%>
</div>
</NeatHtml:UntrustedContent>

Basically I wanted to force images to a certain size, regardless of what the user put into the excerpt, and to shrink up the div a bit, as neathtml was putting a empty div at the end of each feed. Here is what I added:

.rssTextNeatHtml img { height: 100px; width: 100px; } /*tim c: force the images to a set size, regardless of size in excerpt*/
.rssTextNeatHtml { margin-bottom: -12px}
 

I also did not want my changes to affect feeds that were not displaying the text.

Oddly enough, that is what I did at 1st. I added that css markup to the rsstext class. However it shrunk feed displays that were only displaying the header making them overlap. I then tried adding a css class to the neathtml control, then a client side style attribute. Both of those caused the control to throw an exception. I did not verify, but I suspect their control is not inherited from WebControl, but more likely Control.  So then I was forced to add that inner div to the neathtml control.

I could have used the img resizing css on the rsstext class however.