Not common, and using anything other than 4 digits is not wise if you want universal support especially when travelling - 6 digit cards are not 100% compatible with every ATM/card machine because some (most?) only allow 4 code entry. I've never heard of 10 digits and the compatibility must be even more limited. Where do you live and what happens if you try to use your card at ATM's abroad?

I believe some cash machines in the UK don't require you to press 'enter' after typing in your code, they just 'go' after 4 have been entered.

Mine was 8 until I came to visit certain European country where every single terminal has PIN length capped at 6. The best vacation I ever had.

It's not the case, it depends on the issuing institution.

Also, there is a practical requirement to have it short; as physical merchants value quick processing, and having to type&re-type long passwords delays other people behind you - so they want long PINs to be unpopular.

Great, now I need to change all my pin numbers. :)