What is the maximum URI (or URL) length supported by the ProxySG?

SGOS 6.7.3.4

SGOS 7.x

The maximum URL length is 16KB or roughly 16,000 characters. Any request  URL length more than 16 KB will result in an error with 400 response. Invalid request exception page is returned in such scenario.

For a https request you can allow such requests by disabling the ssl interception for destination server.
For a http request, there is no workaround. Such requests will result in an error with 400 response.