This can be done quite easily. Assuming you have no spaces or carriage returns then you can simply use the 'Length' function. This will count the number of characters, including spaces and carriage returns, in a field.
If you want to see the number of characters in the field simply create a calculated field with the following:
Length ( Field )
And perform a search of "> 15" on the calculated field.
However if you want to have the calculation to be boolean if the field had more than 15 characters then modifiy it slightly to the following
Length ( Field ) > 15
And this will return a 1 in the field if the length is greater than 15.
Finally if you do have any characters like spaces and carriage returns in the field you do not want to count, you will need to remove these using the Substitute function
Length ( Substitute ( Field ; [ " " ; "" ];[ "¶" ; "" ] )
I hope I have not overcomplicated this for you.
Many, many thanks - this worked like a charm!