>So I can change look of all the icons in the toolbar
Yes. You can change the look in these ways: turn them off, on, replace or remove every icon, toggle to grey, display them as text, switch between two icon sets included with the editor (small and large), enable or disable gradients and finally group toolbars into registers.
> get rid of those Explorer drag bars on the side
Yes. You can turn them on, off, move them around, resize and iconify them (so that they are turned into an icon in the toolbar to be out of the way).
> [change] the Windows resize handle
Yes, for text windows. You can revert back to the system resize handle and change the scrollers back, too.
> change the icons in the prefs windows to some nice looking glowicons
Yes, all images in the editor are external files and can be replaced. Images can be turned off completely, too, in case you prefer retro dialogs. It ships with 300+ images, so you would have some work cut out for you.
> and, last but not least, can I use Non-Proportional truetype fonts with it?
Yes. But you must mean Non-Fixed-Width fonts and those are allowed for dialogs only, not for text (which is probably fine for a source code editor since you want to see source code lined up).
To sum it up, you can make it look like CED or ED or like a typical PC IDE. Since all components can be installed individually, that applies to total size, too. It varies from some hundred KB to 100+ MB with the complete SDK.