diff --git a/pdu/submit_test.go b/pdu/submit_test.go
new file mode 100644
index 0000000..c60646b
--- /dev/null
+++ b/pdu/submit_test.go
@@ -0,0 +1,53 @@
+package pdu
+
+import "testing"
+
+// region size
+func TestCalculateSizeTypicalInstance(t *testing.T) {
+	p := &SUBMIT_SM{
+		service_type:           "test_service",
+		source_addr:            "12345",
+		destination_addr:       "67890",
+		schedule_delivery_time: "",
+		validity_period:        "",
+		short_message:          "Hello, World!",
+	}
+	expectedSize := uint32(16 + len(p.service_type) + 1 + 1 + len(p.source_addr) + 1 + 1 + len(p.destination_addr) + 1 + 1 + 1 + len(p.schedule_delivery_time) + len(p.validity_period) + 1 + 1 + 1 + 1 + 1 + len(p.short_message))
+	actualSize := p.Size()
+	if actualSize != expectedSize {
+		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
+	}
+}
+
+func TestCalculateSizeMaxLengths(t *testing.T) {
+	maxLen := 255
+	p := &SUBMIT_SM{
+		service_type:           string(make([]byte, maxLen)),
+		source_addr:            string(make([]byte, maxLen)),
+		destination_addr:       string(make([]byte, maxLen)),
+		schedule_delivery_time: string(make([]byte, maxLen)),
+		validity_period:        string(make([]byte, maxLen)),
+		short_message:          string(make([]byte, maxLen)),
+	}
+	expectedSize := uint32(16 + maxLen + 1 + 1 + maxLen + 1 + 1 + maxLen + 1 + 1 + 1 + maxLen + maxLen + 1 + 1 + 1 + 1 + 1 + maxLen)
+	actualSize := p.Size()
+	if actualSize != expectedSize {
+		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
+	}
+}
+
+func TestHandlesEmptyStringsForAllStringFields(t *testing.T) {
+	p := &SUBMIT_SM{
+		service_type:           "",
+		source_addr:            "",
+		destination_addr:       "",
+		schedule_delivery_time: "",
+		validity_period:        "",
+		short_message:          "",
+	}
+	expectedSize := uint32(16 + len(p.service_type) + 1 + 1 + len(p.source_addr) + 1 + 1 + len(p.destination_addr) + 1 + 1 + 1 + len(p.schedule_delivery_time) + len(p.validity_period) + 1 + 1 + 1 + 1 + 1 + len(p.short_message))
+	actualSize := p.Size()
+	if actualSize != expectedSize {
+		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
+	}
+}