Because I have a single-monitor laptop I dock my panels (floating panels just get in the way). I'm using the gtkrc-hidpi theme because I have a 200% DPI scaling in Windows 8.1. However the docked panels can't be shrunken below 1220px (about 45% of my screen width!!). Even when I use the old gtkrc profile, I can't resize the panels to less than 618px on my laptop. On my desktop PC (100% DPI scaling) I can shrink the panel to 384px.

When running Inkscape, I have the Windows 8.1 compatibility option 'Disable High DPI scaling' enabled because without it doubles all pixels (even of the drawing). I tried all of v0.91 32bit, 64bit and portable and v0.48 32-bit.

Try a clean install, find your inkscape folder in your user folder and delete the whole folder, find the gtk folder and delete that too.
I haven't used gtk themes for a while so I'm not sure how this might affect other programs that use gtk if you have such
I'm using inkscape on 8.1 without any special options but no theme, so perhaps you need to look at how badly you want that theme