Resolution problem

I am running parallels 10 on mac OS X 10.11.6
I was using vista which worked fine.
Since installing windows 7 the text on some programmes is too small.
I have followed the advice given online but nothing seems to work.
The screen resolution is set to 2560 x 1396
When I attempt to change it to the recommended setting of 1024 x 768 it reverts to the prior setting, despite my choosing 'keep settings'.
Please advise
Norman