Hey! you're in the right place you have to edit both
here's what I did to mine wide screen I use (1280x1024)
root-extra-themes-default-theme.plist
hope this will help you.
you need to edit these string: the red text are the new string
<key>Enabled</key>
<string>no</string>
<key>New item</key>
<string></string>
<key>background_pos_x</key>
<string>50%</string>
<key>background_pos_y</key>
<string>0</string>
<key>boot_bgcolor</key>
<string>#AAAAAA</string>
<key>boot_height</key>
<string>1024</string> <key>boot_width</key>
<string>1280</string> <key>bootprompt_bgcolor</key>
<string>#222334</string>
<key>bootprompt_height</key>
<string>20</string>
<key>bootprompt_pos_x</key>
<string>%50</string>
<key>bootprompt_pos_y</key>
<string>-2</string>
<key>bootprompt_textmargin_h</key>
<string>10</string>
<key>bootprompt_textmargin_v</key>
<string>5</string>
<key>bootprompt_transparency</key>
<string>1</string>
<key>bootprompt_width</key>
<string>50%</string>
<key>countdown_pos_x</key>
<string></string>
<key>countdown_pos_y</key>
<string>-30%</string>
<key>devices_bgcolor</key>
<string>#222334</string>
<key>devices_iconspacing</key>
<string>20</string>
<key>devices_layout</key>
<string>horizontal</string>
<key>devices_max_visible</key>
<string>5</string>
<key>devices_pos_x</key>
<string></string>
<key>devices_pos_y</key>
<string></string>
<key>devices_transparency</key>
<string>0</string>
<key>infobox_bgcolor</key>
<string>#222334</string>
<key>infobox_height</key>
<string>406</string>
<key>infobox_pos_x</key>
<string></string>
<key>infobox_pos_y</key>
<string></string>
<key>infobox_textmargin_h</key>
<string>10</string>
<key>infobox_textmargin_v</key>
<string>10</string>
<key>infobox_transparency</key>
<string>10</string>
<key>infobox_width</key>
<string>650</string>
<key>logo_bgcolor</key>
<string>#000000</string>
<key>logo_pos_x</key>
<string></string>
<key>logo_pos_y</key>
<string>3%</string>
<key>logo_transparency</key>
<string>255</string>
<key>menu_bgcolor</key>
<string>#222334</string>
<key>menu_pos_x</key>
<string></string>
<key>menu_pos_y</key>
<string>-5%</string>
<key>menu_textmargin_h</key>
<string>10</string>
<key>menu_textmargin_v</key>
<string>5</string>
<key>menu_transparency</key>
<string>1</string>
<key>progressbar_height</key>
<string>40</string>
<key>progressbar_pos_x</key>
<string></string>
<key>progressbar_pos_y</key>
<string>-37%</string>
<key>progressbar_width</key>
<string>100</string>
<key>screen_bgcolor</key>
<string>#222334</string>
<key>screen_height</key>
<string>1024</string> <key>screen_textmargin_h</key>
<string>10</string>
<key>screen_textmargin_v</key>
<string>10</string>
<key>screen_width</key>
<string>1280</string>QUOTE (filip95 @ Apr 13 2009, 09:39 PM)

Hello,
I have searched for nearly 2 hours now, but couldn't find anything regarding the question I have. I would like to get 1024x600 at the Chameleon menu if possible. The video card is an onboard one - a Intel GMA 950 (Chameleon lists it as "Intel® 82495GM Chipset Family Graphics Chip Accelerated VGA BIOS". I have already tried to edit the resolution in the /Extra/com.apple.Boot.plist and /Extra/Themes/<ThemeName>/Theme.plist files - didn't work, I still get 800x600 in the menu.
--filip95