Hello. I had the same problem since I updated iTunes a few days ago: everything was huge, there was no side menu on the right, etc.
I found this thread half an hour ago and I saw I wasn't the only one with that issue.
BTW, my PC has Windows 7, 1920 x 1080 screen resolution, its DPI is set on 125% (I hate tiny pixelated fonts) and also I use that font renderer called GDIPP to make fonts look way better than the crappy ClearType (the programs are called gdipp_x64_0.9.1 and gdipp_x86_0.9.1, both are installed on my PC but I don't remember where I downloaded them from).
This GDIPP had nothing to do with the issue, of course (and I highly recommend it if you like non-pixelated and nice rendered fonts on the whole Windows system).
Anyway, I tried going back to 100% DPI and it solved the look and functions of iTunes but, as I said, I can't use Windows with those tiny ugly fonts.
A few minutes ago I tried this: right click, screen resolution, reduce or enlarge (or something like that), I kept the 125% DPI, clicked on the left side menu on PPP, unchecked that XP style option, applied changes and... voilà!
I can't believe I found the solution all by myself!
I wonder what consequences will I face by having that XP option unchecked... everything looks fine so far (Windows, iTunes and other programs too).
Try my suggestions (including GDIPP) and let me know if it worked for you too.
See ya!