I think this isn't possible. Application isn't able to control this. Imagine what a mess would be in system tray if application would choose which icon to use. And it doesn't depend on taskbar size too.
Now a little bit about links which made me believe in all I said:
http://msdn.microsoft.com/en-us/library/bb773352(v=vs.85).aspx[
^]
It is about structure in Win API which defines icon in system tray:
"If only a 16x16 pixel icon is provided, it is scaled to a larger size in a system set to a high dpi value. This can lead to an unattractive result. It is recommended that you provide both a 16x16 pixel icon and a 32x32 icon in your resource file."
And
http://msdn.microsoft.com/en-us/library/aa511448.aspx[
^]
Which also says "Use an icon with 16x16, 20x20, and 24x24 pixel versions. The larger versions are used in high-dpi display modes."
It looks like this is some system settings or just system somehow evaluates which icon to use.