I was looking into this also. The only way to do it that I’ve found is to edit an HTML file within the plugin itself. It’s called email_template.html.
/wp-content/plugins/wysija-newsletters/tools/templates/newsletter/email/email_template.html
This is use at your own risk. I haven’t fully vetted it but it seems to work just fine. And all footer images etc would have to match. But it would needed to be changed every time you update the plugin. And if you have something comparing the files to the repository like WordFence you would need to keep that in mind because it will likely flag it as a problem.
This really should be a setting option somewhere as a variable. 600 px is not a one size fits all.
I changed the width on all these….900 was 600….
<table width="100%" cellpadding="0" cellspacing="0" border="0" id="wysija_wrapper">
<tr>
<td valign="top" align="center">
<table width="900" cellpadding="0" cellspacing="0" border="0" align="center">
{if ! #hide_viewbrowser}
<tr>
<td width="900" style="min-width:900px;" valign="top" align="center" {if #is_rtl} dir="rtl" {/if} >
{#viewbrowser}
</td>
</tr>
{/if}
<tr>
<td width="900" style="min-width:900px;" valign="top" align="center">
{#header}
</td>
</tr>
<tr>
<td width="900" style="min-width:900px;" valign="top" align="left">
{#body}
</td>
</tr>
<tr>
<td width="900" style="min-width:900px;" valign="top" align="center" {if #is_rtl} dir="rtl" {/if} >
{#footer}
</td>
</tr>
{if ! #hide_unsubscribe}
<tr>
<td width="900" style="min-width:900px;" valign="top" align="center" {if #is_rtl} dir="rtl" {/if} >
{#unsubscribe}
</td>
</tr>
{/if}
</table>
</td>
</tr>
</table>