What's the best setting of iPad for senior grandma?

There is a maximum text size when changing the text size from Settings → Display & Brightness → Text Size, however larger text sizes are available. To enable these larger sizes, go to Settings → General → Accessibility → Larger Text and enable Larger Accessibility Sizes, then increase the text size even further using the slider.

To increase the contrast of certain elements which may make the text easier to read further, go to Settings → General → Accessibility → Increase Contrast and enable Reduce Transparency and Darken Colors.

You can also enable the Zoom function (also under Accessibility) which enables a three-finger double-tap for zooming in on the screen.

Furthermore, the Colour Filters allow for further customisation of the colours displayed. Accessible from Settings → General → Accessibility → Display Accommodations → Colour Filters, they "can be used to … aid users who have difficulty reading text on the display".

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A good way to find such settings is by using the search functionality in the Settings app.