I can use fn:length(mystring) to find the length/number of character of a string.
fn:length(mystring)
How can I find the length/number of digits in an integer?
e.g. if my int is 3000, I want to return 4.
Convert it to String first by evaluating it as body of <c:set>:
String
<c:set>
<c:set var="intAsString">${someInt}</c:set>
Then you can get its length the usual way:
${fn:length(intAsString)}