Upgrade to Windows 10. Windows 10 taskbar icon size is perfect.
I’ve tried a couple of customizing apps that seemed like they’d be able to do custom taskbar icon size (WinAero Tweaker, Actual Multiple Monitors), but they have no such option. So Windows 10 is all that I can say improves this from experience.
However, Windows 10 has some bugs that are not in 8.1. For example, I get DPC latency every 30 minutes, which causes one second of audio and video stutter. (Even though I updated BIOS and drivers frequently. This seems to be a quite common problem, although it doesn’t affect everybody with Win 10.)
So if you are willing to take some risk, Windows 10 definitely has a look that’s more optimized for desktop PCs than Windows 8.1.