add payload calculation

test/loadtime/payload/size.go

@@ -0,0 +1,23 @@
+package payload
+
+import (
+	"math"
+
+	"google.golang.org/protobuf/proto"
+	timestamppb "google.golang.org/protobuf/types/known/timestamppb"
+)
+
+func CalculateUnpaddedSizeBytes() (int, error) {
+	p := &Payload{
+		Time:        timestamppb.Now(),
+		Connections: math.MaxUint64,
+		Rate:        math.MaxUint64,
+		Size:        math.MaxUint64,
+		Padding:     make([]byte, 1),
+	}
+	b, err := proto.Marshal(p)
+	if err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}

test/loadtime/payload/size_test.go

@@ -0,0 +1,19 @@
+package payload_test
+
+import (
+	"testing"
+
+	"github.com/tendermint/tendermint/test/loadtime/payload"
+)
+
+const payloadSizeTarget = 1024 // 1kb
+
+func TestCalculateSize(t *testing.T) {
+	s, err := payload.CalculateUnpaddedSizeBytes()
+	if err != nil {
+		t.Fatalf("calculating unpadded size %s", err)
+	}
+	if s > payloadSizeTarget {
+		t.Fatalf("unpadded payload size %d exceeds target %d", s, payloadSizeTarget)
+	}
+}