I'm using IPCC 7.x and use a handful of length expressions.
I would accomplish this by adding an 'If' node and using the length expression like this:
len(CallerEnteredDigits)=5
The success result would be exactly 5 digits in CED, a failure would be anything other than 5 digits.
You can manipulate that further by using greater/less than. You could also plug any variable into the quotes to perform the length function against it the same way.
Hope this helps.