Following Raichu's suggestion, I have created a simple "mod" to the forum's styles, which enables a fluid layout (adapts to your browser window size).

Steps:

1. This works only in the Firefox browser (and variants like Flock etc.).
2. Install the Stylish extension for Firefox.
3. After you restart Firefox to activate the extension you will see a paintbrush-like icon at the bottom of the window. Click it once, select Write Styles... > For forum.koohii.com. In the new window, enter a description eg. "RevTK Forums Fluid Layout". In the large textbox, copy and paste the following:

---------------copy/paste all the blue text----------------

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("forum.koohii.com") {

#container { margin:0 !important; width:100% !important; border-left:0 !important; border-right:0 !important; }
#container #body { width:100% !important; padding-left:0 !important; padding-right:0 !important; }
#container #body #punwrap { margin:0 20px !important; }
#footer { margin:0 !important; width:100% !important; }

/* header */
#header ul.primary { width:auto !important; }

.topsection { width:99% !important; padding:20px 0 1em !important; }
#pbb .topnav { margin:0 20px 0 0 !important; }
#pbb .activetopics { width:auto !important; float:left !important; }
#pbb .topic .header { margin-left:220px !important; float:none !important; width:auto !important; }

/* If  you set margin-left to "0", the text will wrap around the author boxes */
#pbb .topic .message { margin-left:220px !important; float:none !important; width:auto !important; }

/* Index */
#punindex #content { max-width:600px !important; }

/* Here you can increase the font size in posts */
#pbb .message, #pbb .message p { font-size:13px !important; }

#pbb #sidebar.roundbox { position:absolute !important; }

#pbb #content { float:none !important; width:auto !important; margin-left:220px !important; }
#pbb .viewlist_pager { clear:right !important; }
#pbb #content.viewforum .thread { width:100%; clear:both !important; }

/*Profile/Options*/
#pbb .block2col div.blockform, #pbb div.block2col div.block { float:none !important; width:auto !important; margin-left:220px !important; }

/* Post Topic */
#pbb .posttopic_main { float:none !important; width:auto !important; margin-left:220px !important; }
#pbb .posttopic_post { background:#EEECD8 !important; float:none !important; width:auto !important; margin-left:220px !important; }

#pbb .posttopic_post textarea { width:96% !important; }


}

---------------copy/paste up to line above----------------

4. Click Save.

That's it.

You delete the mod if you want to revert to the original "fixed layout" version of this forum. But you can also temporarily disable the fluid layout if you need to: go to Manage Styles and then double-click the styles you've just added. You will see a checkbox option that lets you disable/enable the "mod".