diff --git a/sds/command.go b/sds/command.go
index 0f0fae9..194453f 100644
--- a/sds/command.go
+++ b/sds/command.go
@@ -1,7 +1,11 @@
 package sds
 
 import (
+	"context"
 	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
 
 	"github.com/ftl/tetra-pei/tetra"
 )
@@ -16,17 +20,50 @@ func (f EncoderFunc) Encode() ([]byte, int) {
 	return f()
 }
 
+type RequesterFunc func(context.Context, string) ([]string, error)
+
 const (
-	// SwitchToSDSTL is a short-cut for selecting the SDS-TL AI service according to [PEI] 6.14.6
+	// CRLF line ending for AT commands
+	CRLF = "\x0d\x0a"
+	// CtrlZ line ending for PDUs
+	CtrlZ = "\x1a"
+
+	// SwitchToSDSTL is a short-cut for selecting the SDS-TL AI service with ISSI addressing and E2EE according to [PEI] 6.14.6
 	SwitchToSDSTL = "AT+CTSDS=12,0,0,0,1"
-	// SwitchToStatus is a short-cut for selecting the status AI service according to [PEI] 6.14.6
+	// SwitchToStatus is a short-cut for selecting the status AI service with ISSI addresssing according to [PEI] 6.14.6
 	SwitchToStatus = "AT+CTSDS=13,0"
 )
 
 // SendMessage according to [PEI] 6.13.2
 func SendMessage(destination tetra.Identity, sds Encoder) string {
-	pdu := make([]byte, 0, 2000) // TODO use the maximum size allowed
+	pdu := make([]byte, 0, 256)
 	pduBits := 0
 	pdu, pduBits = sds.Encode(pdu, pduBits)
-	return fmt.Sprintf("AT+CMGS=%s,%d\x0d\x0a%s\x1a", destination, pduBits, tetra.BinaryToHex(pdu))
+	return fmt.Sprintf("AT+CMGS=%s,%d"+CRLF+"%s"+CtrlZ, destination, pduBits, tetra.BinaryToHex(pdu))
+}
+
+var sendMessageDescription = regexp.MustCompile(`^\+CMGS: .+\(\d*-(\d*)\)$`)
+
+// RequestMaxMessagePDUBits uses the given RequesterFunc to find out how many bits a message PDU may have (see [PEI] 6.13.2).
+func RequestMaxMessagePDUBits(ctx context.Context, requester RequesterFunc) (int, error) {
+	responses, err := requester(ctx, "AT+CMGS=?")
+	if err != nil {
+		return 0, err
+	}
+	if len(responses) < 1 {
+		return 0, fmt.Errorf("no response received")
+	}
+	response := strings.ToUpper(strings.TrimSpace(responses[0]))
+	parts := sendMessageDescription.FindStringSubmatch(response)
+
+	if len(parts) != 2 {
+		return 0, fmt.Errorf("unexpected response: %s", responses[0])
+	}
+
+	result, err := strconv.Atoi(parts[1])
+	if err != nil {
+		return 0, err
+	}
+
+	return result, nil
 }
diff --git a/sds/command_test.go b/sds/command_test.go
index bc13b40..8f31058 100644
--- a/sds/command_test.go
+++ b/sds/command_test.go
@@ -1 +1,45 @@
 package sds
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRequestMaxPDUBits(t *testing.T) {
+	tt := []struct {
+		desc     string
+		response []string
+		expected int
+		invalid  bool
+	}{
+		{
+			desc:    "empty",
+			invalid: true,
+		},
+		{
+			desc: "happy path",
+			response: []string{
+				"+CMGS: (0-16777214,00000001-10231638316777214,1-255,0-999999999999999999999999),(8-1184)",
+				"",
+				"OK",
+			},
+			expected: 1184,
+		},
+	}
+	for _, tc := range tt {
+		t.Run(tc.desc, func(t *testing.T) {
+			requester := func(_ context.Context, _ string) ([]string, error) {
+				return tc.response, nil
+			}
+			actual, err := RequestMaxMessagePDUBits(context.Background(), requester)
+			if tc.invalid {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tc.expected, actual)
+			}
+		})
+	}
+}