mac on Limits with the length of "uri" when using Ciao

What limits apply to the length of "uri" when using Ciao to send a URL to ThingSpeak (free account)?

CiaoData data = Ciao.write(CONNECTOR, SERVER_ADDR, uri); // to send the URL to ThingSpeak?

I am using a Arduino Uno with ESP8266 WiFi built in (not a WiFi Shield) to send Temperature/Humidity data from DHTxx sensors.

My code works with 2 sensors (4 data fields) but fails with 3 sensors.

#define CONNECTOR "rest"
#define SERVER_ADDR "api.thingspeak.com"

#define APIKEY_THINGSPEAK   "my key"

String uri = "/update?api_key=";
uri += APIKEY_THINGSPEAK;
uri += "&field1=";
uri += String(h1);
uri += "&field2=";
uri += String(t1);
uri += "&field3=";
uri += String(h2);
uri += "&field4=";
uri += String(t2);

CiaoData data = Ciao.write(CONNECTOR, SERVER_ADDR, uri);

Would it better to use

ThingSpeak.writeField(myChannelNumber,1,t1, myWriteAPIKey);

ThingSpeak.writeField(myChannelNumber,2,t2, myWriteAPIKey);

etc