diff --git a/source/include/cellular_config_defaults.h b/source/include/cellular_config_defaults.h
index fd1d8cc8..320af51a 100644
--- a/source/include/cellular_config_defaults.h
+++ b/source/include/cellular_config_defaults.h
@@ -440,6 +440,18 @@
       ( !( CELLULAR_CHECK_IS_PREFIX_LEADING_CHAR( inputChar ) ) ) )
 #endif
 
+/**
+ * @brief Cellular AT string length.<br>
+ *
+ * The maximum length of an AT string.<br>
+ *
+ * <b>Possible values:</b>`Any positive integer`<br>
+ * <b>Default value (if undefined):</b> 256
+ */
+#ifndef CELLULAR_AT_MAX_STRING_SIZE
+    #define CELLULAR_AT_MAX_STRING_SIZE    ( 256U )
+#endif
+
 /**
  * @brief Macro that is called in the cellular library for logging "Error" level
  * messages.
diff --git a/source/include/common/cellular_at_core.h b/source/include/common/cellular_at_core.h
index 68d08a3b..bfe094db 100644
--- a/source/include/common/cellular_at_core.h
+++ b/source/include/common/cellular_at_core.h
@@ -45,17 +45,14 @@
 /* Standard includes. */
 #include <stdint.h>
 
-/*-----------------------------------------------------------*/
-
-/**
- * @brief Maximum size of an AT string.
- */
-#define CELLULAR_AT_MAX_STRING_SIZE    ( 256U )
+/* Cellular includes. */
+#ifndef CELLULAR_DO_NOT_USE_CUSTOM_CONFIG
+    /* Include custom config file before other headers. */
+    #include "cellular_config.h"
+#endif
+#include "cellular_config_defaults.h"
 
-/**
- * @brief Maximum size of an AT prefix.
- */
-#define CELLULAR_AT_MAX_PREFIX_SIZE    ( 32 )
+/*-----------------------------------------------------------*/
 
 /**
  * @brief The array size of an array.
diff --git a/test/unit-test/cellular_at_core_utest.c b/test/unit-test/cellular_at_core_utest.c
index 7accfd06..42cf7377 100644
--- a/test/unit-test/cellular_at_core_utest.c
+++ b/test/unit-test/cellular_at_core_utest.c
@@ -888,6 +888,20 @@ void test_Cellular_ATStrStartWith_Empty_Prefix( void )
     TEST_ASSERT_EQUAL( CELLULAR_AT_BAD_PARAMETER, cellularStatus );
 }
 
+/**
+ * @brief Test the long string case for Cellular_ATStrStartWith to return CELLULAR_AT_BAD_PARAMETER.
+ */
+void test_Cellular_ATStrStartWith_ATStringTooLong( void )
+{
+    CellularATError_t cellularStatus = CELLULAR_AT_SUCCESS;
+    bool result;
+    char * pPrefix = "";
+    char pStringSuccess[] = CELLULAR_SAMPLE_PREFIX_STRING_LARGE_INPUT;
+
+    cellularStatus = Cellular_ATStrStartWith( pStringSuccess, pPrefix, &result );
+    TEST_ASSERT_EQUAL( CELLULAR_AT_BAD_PARAMETER, cellularStatus );
+}
+
 /**
  * @brief Test that any NULL parameter causes Cellular_ATStrtoi to return CELLULAR_AT_BAD_PARAMETER.
  */