The older iPad's have a 9,7" 1024×768 screen with 132 pixels per inch, and the newer 'Retina' ones have same sized screens but it's a 2048×1536 panel with 264ppi, so it's a lot sharper.
And with the Mini's its 7,9" screens with the same resolutions so 163ppi in the older one and 326ppi in the newer one.
At about 300ppi it becomes impossible for human eye to distinguish pixels from another.