The default request size Hiawatha accepts is 64kb. If you want to increase this, use the MaxRequestSize option in httpd.conf. Example:
Port {
...
MaxRequestSize = 1024
}
makes Hiawatha accept HTTP requests up to 1MB. Be carefull with this setting. If you set it to high, an attacker can misuse it to DoS your server (make it run out of memory).
See the Hiawatha manualpage for more information about Hiawatha configuration options.