Hello Frank,
The maximum URL length in NW AS ABAP is hard-coded to 4096. This is seen as a reasonable approach since most Web Browsers have difficulties themselves with very long URLs. You can find an extensive discussion here:
http://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers
In your case, you probably do not have a browser on the client side. However, I think it is good practice in general to avoid lengthy URLs since they might also cause issues with intermediary devices such as proxys, load balancers etc.
Regards,
Tobias