Instead of adding an ErrorType() method, I think implementing the Unwrap interface to return MessageSizeTooLarge would be better, then you can just call errors.Is with the wrapped error:

diff --git a/error.go b/error.go
index 7fbf3a732215..7db007a2ab50 100644
--- a/error.go
+++ b/error.go
@@ -656,16 +656,7 @@ func (e MessageTooLargeError) Error() string {
        return MessageSizeTooLarge.Error()
 }

-// ErrorType returns the specific error type associated with the MessageTooLargeError.
-// This function returns the predefined Error constant MessageSizeTooLarge, indicating
-// that the error occurred due to a message being too large to fit within the allowed size.
-//
-// Example:
-//
-//     err := MessageSizeTooLarge
-//     msgTooLarge := []Message{...}
-//     errors.Is(err, msgTooLarge.ErrorType())
-func (e MessageTooLargeError) ErrorType() Error {
+func (e MessageTooLargeError) Unwrap() error {
        return MessageSizeTooLarge
 }

diff --git a/error_test.go b/error_test.go
index cea810b15739..811243fe6d93 100644
--- a/error_test.go
+++ b/error_test.go
@@ -1,7 +1,6 @@
 package kafka

 import (
-       "errors"
        "fmt"
        "testing"

@@ -121,8 +120,8 @@ func TestError(t *testing.T) {
                        {Key: []byte("key"), Value: make([]byte, 1024*1024)},
                }
                msgTooLarge := messageTooLarge(msg, 1)
-               assert.False(t, errors.Is(err, msgTooLarge))
-               assert.Equal(t, err.Error(), msgTooLarge.Error())
-               assert.True(t, errors.Is(err, msgTooLarge.ErrorType()))
+               assert.NotErrorIs(t, err, msgTooLarge)
+               assert.Contains(t, msgTooLarge.Error(), MessageSizeTooLarge.Error())
+               assert.ErrorIs(t, msgTooLarge, MessageSizeTooLarge)
        })
 }

Instead of adding an ErrorType() method, I think implementing the Unwrap interface to return MessageSizeTooLarge would be better, then you can just call errors.Is with the wrapped error:

diff --git a/error.go b/error.go
index 7fbf3a732215..7db007a2ab50 100644
--- a/error.go
+++ b/error.go
@@ -656,16 +656,7 @@ func (e MessageTooLargeError) Error() string {
        return MessageSizeTooLarge.Error()
 }

-// ErrorType returns the specific error type associated with the MessageTooLargeError.
-// This function returns the predefined Error constant MessageSizeTooLarge, indicating
-// that the error occurred due to a message being too large to fit within the allowed size.
-//
-// Example:
-//
-//     err := MessageSizeTooLarge
-//     msgTooLarge := []Message{...}
-//     errors.Is(err, msgTooLarge.ErrorType())
-func (e MessageTooLargeError) ErrorType() Error {
+func (e MessageTooLargeError) Unwrap() error {
        return MessageSizeTooLarge
 }

diff --git a/error_test.go b/error_test.go
index cea810b15739..811243fe6d93 100644
--- a/error_test.go
+++ b/error_test.go
@@ -1,7 +1,6 @@
 package kafka

 import (
-       "errors"
        "fmt"
        "testing"

@@ -121,8 +120,8 @@ func TestError(t *testing.T) {
                        {Key: []byte("key"), Value: make([]byte, 1024*1024)},
                }
                msgTooLarge := messageTooLarge(msg, 1)
-               assert.False(t, errors.Is(err, msgTooLarge))
-               assert.Equal(t, err.Error(), msgTooLarge.Error())
-               assert.True(t, errors.Is(err, msgTooLarge.ErrorType()))
+               assert.NotErrorIs(t, err, msgTooLarge)
+               assert.Contains(t, msgTooLarge.Error(), MessageSizeTooLarge.Error())
+               assert.ErrorIs(t, msgTooLarge, MessageSizeTooLarge)
        })
 }

Great recommendation! Implementation method has been updated.

@AndrewShearBayer & @jkoelker : any concern with me merging this?

No concerns here!