I would go for the smaller one; 27" is really huge and the pixel really small. I think the 21"fit better.
I need to replace a 1st gen 17" intel imac that my 80 year mother has used for the last 5 years.
What is the best size for the elderly? A 27 " or 21.5"? I guess Universal Access is must for both resolutions.
However, you can buy the 27", and turn it into "HiDPI" mode. That is the technology behind the Retina display, just without an actual Retina display. The iMac will behave as if it had 1280 x 720 pixels on 27", but with the same extra quality for text and graphics as a retina display.
I agree with this, get the 27 unless it can't possibly be made to fit her space.This This This This. I have had double cataract surgery. I did this with my 27 inch iMac which I kept for 18 months before selling.
I set the screen at 720p and all was well. Go to an apple store and ask a genius to do this on a store iMac. Takes seconds to do and undo.