FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 198607
This article gives a useful tip to increase the max request length that can be requested by client workstations when using WAD explicit proxy.

When using the FortiGate as an explicit proxy, some users might encounter websites on which they are not able to POST their data, like file/comment on Facebook for example.

To increase the maximum request length inside the web proxy, configure the following CLI settings:
#config web-proxy global
    set max-request-length <VALUE>
By default, max-request-length is 4 Kbytes.

Starting from FortiOS 5.6.11, the default value has been increased to 8 Kbytes.

The value can be configured between 4 and 64 Kbytes.