While I'm not an experienced skin coder by any means, the quickest solution that comes to mind is setting the entire site's width to the width you'd like for the banner and navigation, then manually editing everything else to be smaller.

For example:

If you set the entire site's width to 1000px, then when editing the coding for everything else (boards, info center, etc, etc) put them at a lesser width.

So instead of putting:

.boardtitlebar { width:100%/1000px } you put .boardtitlebar { width:800px; } in the style sheet.

If you don't make your own codes for the different areas of the skin, you can always try playing around with what already exists in the style sheet. I think each section have their own section for width and stuff (or so I assume?).

Quick clarifying question if the above doesn't help,

Are you trying to make the navbar (the menu bar, login/home/etc) a different size while in the main wrapper?

Hmmmm if I may link an example? I'm having trouble describing and wording what I'm talking about ;o;

only-teardrops.proboards.com/
A bit like Alcove's skin here? Like not exactly the same, but I basically want the banner and nav stuff to fit the length of the forum, while leaving the boards more narrow. They'd all be attached the same way since the BG is meant to be involved in the banner....I'm so bad at wording coding and skinning things oh dear...

It's okay!

What's happening on Alcove's skin there - is the whole header is outside of the main wrapper. In both jcink and proboards, they have the #wrapper- which holds the boards and the navtree.

What you want is to move all of that outside the main wrapper and simply just sit it on top of the wrapper. You cannot edit the size of the nav bar to go all the way across inside the main wrapper. From what I'm guessing they simply created the transparent image in the middle (The picture) and put all of that the same size of the forum and just auto-centered it- but the nav bar is going all the way across.

Check out alcove's skin with the right click + inspect for a quick look!

For example, take

<a id="navigation-skip" href="#content" accesskey="s" class="aria-hidden" title="Skip Navigation">Skip Navigation</a>
<a href="#" accesskey="d" title="Open Menu" onclick="proboards.hotkeys.activate(Keys.d); return false;"></a>
<div id="navigation-menu" class="ui-helper-clearfix">
<ul role="navigation">
{foreach $[navigation.menu]}
<li>
<a{if $[navigation.menu.active]} class="state-active"{/if} href="$[navigation.menu.href]"{if $[navigation.menu.accesskey]} accesskey="$[navigation.menu.accesskey]"{/if}>
$[navigation.menu.name]
{if $[navigation.menu.notification.total]}
<div class="tip-holder" onclick="window.location='$[navigation.menu.notification.href]'; return false;">
<div class="tip-number">$[navigation.menu.notification.total]</div>
<span class="tip"></span>
</div>
{/if}
</a>
</li>
{/foreach}
</ul>
<p id="welcome">
{if !$[current_user.is_member]}
Welcome Guest.
{if $[login_link] || $[register_link]}
Please $[login_link]{if $[login_link] && $[register_link]} or {/if}$[register_link].
{/if}
{else}
<span>Welcome $[current_user.name].</span>  $[logout_link]
{/if}
</p>
</div>

and move it above line 14 on the normal default, not sure if you've got something different there though.

OH BLESS THANK YOU, that did exactly what I wanted it to do!

No problem! Feel free to post any other issues you run into here and someone should help you out! There's also the coding spot in discord, or feel free to hit me up in PM. <3