Q: Partitioning a 3TB Fusion Drive

You may to read the article linked below before proceeding.

http://blog.twocanoes.com/post/55200559532/how-to-mess-up-your-fusion-drive-a-ca utionary

Check the FAQs at > Mac mini (Late 2012 and later), iMac (Late 2012 and later): About Fusion Drive - Apple Support

But with the 5K retina display I have, all the text, icons and alike in Windows appears quite tiny. I was told by a senior Apple Tech that there is no way to get around it.

They are tiny because each pixel on these screens is super tiny. There is one way, but you give up a bit of the crispness of the screen image/text: go to System Preferences > Displays > and take down the resolution by a notch or two (choose the Scaled button and make your choices). That will make everything incrementally larger and I don't believe it would be different with Windows - it is related to the many thousands of pixels that make up your display.

Also, you can to go Finder > View > View Options and increase the text size - that will change all Finder windows, etc. It will not change the menu bar though. And, you'll have to set the text sizes in all apps with text size options.