The vast majority of visually impaired people are not literally blind and that's really important when designing for the general population. If you force everybody whose sight has deteriorated to the point where they can't read text to resort to feeling note sizes you're missing a vast middle ground of people who'd have been served by just choosing a different colour for each denomination.

The new polymer notes do include considerations for the vision-impaired.

Check out the "tactile feature" section on the various banknotes at https://www.bankofengland.co.uk/banknotes/current-banknotes

While we don't use tactile features in Australia, similar consultation went into the design of our banknotes: https://www.rba.gov.au/publications/bulletin/2015/mar/1.html