I've been testing plasma 5 on my 2 boxes running linux mint 17.
Both boxes are basically identical set up, i5-2400, 8gb ram, only the gfx card is different.
One is a passive cooled HD 6600 (my office pc) the other one runs a R9 280x with 3 gb ram.

On the gaming machine I experience crashes of plasma 5 (ofc, it's unstable), but it leads inevitable to a situation where I end with only part of the system settings being left to change, icon size in main menu, control bar and desktop as well as lock screen are hugely increased to the point of unusability.

I have not found a way to reverse this other than to purge, delete remaining configs manually, reinstall, wait for it to happen again.
No issues whatsoever with the old plasma, nor on the other box.

I read that there was some issue with increased icon size on KDE 4 that was supposed to be a feature, but I've not been able to relay this to plasma 5.

edit: forgot to mention gfx driver
fglrxinfo
display: :0 screen: 0
OpenGL vendor string: Advanced Micro Devices, Inc.
OpenGL renderer string: AMD Radeon HD 7900 Series
OpenGL version string: 4.4.13084 Compatibility Profile Context 14.301.1001

I would suggest, once tihs occurs again, to file a bug on bugs.kde.org, and add your configuration files when it happens (along with a screenshot displaying the problem).