Just in case anybody else has this problem with Windows 8, I finally solved it on my PC (ridiculously simply and easily).
I’ll give you the exact steps I tried (although I suspect step number one isn’t necessary - you can probably just start Daz normally).
1. Start Daz Studio by SHIFT right clicking the desktop icon and choosing ‘Open’.
2. It’ll start minimized in the taskbar as always, but SHIFT right click over the taskbar icon and choose ‘Restore’.
3. The program will maximise. In my case, it was all squashed into one narrow panel on the right side of the screen - just drag it out and make it full screen, close it, and then every time thereafter, it will open normally, full screen, without any fuss,
The key for me was the shift right click which offered different menu options I’d never seen before.
Maximise and restore using task manager didn’t work. (Running it in compatibility mode didn’t help either, but thank you for the suggestion.)