The length of an HTTP GET request is not enforced by RFC2616, as Microsoft reports for its IE max length support page.
So, the maximum GET length is a client (browser) related issue. If your app is used by people you can force to use a given browser then you can simply find what is the length this browser support.
In every case I suggest a look to the Wikypedia page about those browser related issues on the Query string (the part of the request bringing parameters for server side apps, the one following the "?" eventually present in a request.
Of course maybe tomcat will put a limit too, on the server side.
Servers MUST be able to handle the URI
of any resource they serve, and SHOULD
be able to handle URIs of unbounded
length if they provide GET-based forms
that could generate such URIs. A
server SHOULD return 414 (Request-URI
Too Long) status if a URI is longer
than the server can handle (see
so you can easily test if Tomcat has a limit and find out what this limit is simply using different requests starting with a very long one giving the error and going down by one half. Then use bisection method to fast find the exact value.