Interesting question. Documentation doesn't reveal the limit:
If you have access to a truncated bind value in a Purepath, you can copy & count. Please beware that the best way is to look at the code, as for instance two consecutive spaces will count only as one in HTML. This has tricked me in the past in other contexts...
It's a setting of the JDBC Sensor, not visible for the customers, unfortunately. Reach to DTONE via in-product chat to get the value for you.
However, it's a better way to capture the information from the application code and not capture whole bind variables unless it's really necessary.