diff --git a/src/MqttClient.cpp b/src/MqttClient.cpp
index af93a03..dba8947 100644
--- a/src/MqttClient.cpp
+++ b/src/MqttClient.cpp
@@ -29,10 +29,12 @@
   #endif
 #endif
 
-#ifdef __AVR__
-#define TX_PAYLOAD_BUFFER_SIZE 128
-#else
-#define TX_PAYLOAD_BUFFER_SIZE 256
+#ifndef TX_PAYLOAD_BUFFER_SIZE
+  #ifdef __AVR__
+    #define TX_PAYLOAD_BUFFER_SIZE 128
+  #else
+    #define TX_PAYLOAD_BUFFER_SIZE 256
+  #endif
 #endif
 
 #define MQTT_CONNECT      1