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You can use StartIsBack if you are using Windows 8. It has that feature.
It's same on both XP and 7. (16px)

Also, these icons are supplied by programs and not Windows itself. Even 7+tt supports it, other programs need to include bigger icons to show.
Try "tray_clock_fix_width" it's on Advanced settings. 
Yes, It's leaked but it's not beta. It's RTM version which everyone should get on next weeks.
I also support this!