diff --git a/proto/encode.go b/proto/encode.go
index 98410a68..b294392f 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -47,6 +47,26 @@ type MarshalOptions struct {
 	// detail and subject to change.
 	Deterministic bool
 
+	// UseCachedSize indicates that the result of a previous Size call
+	// may be reused.
+	//
+	// Setting this option asserts that:
+	//
+	// 1. Size has previously been called on this message with identical
+	// options (except for UseCachedSize itself).
+	//
+	// 2. The message and all its submessages have not changed in any
+	// way since the Size call.
+	//
+	// If either of these invariants is broken, the results are undefined
+	// but may include panics or invalid output.
+	//
+	// Implementations MAY take this option into account to provide
+	// better performance, but there is no guarantee that they will do so.
+	// There is absolutely no guarantee that Size followed by Marshal with
+	// UseCachedSize set will perform equivalently to Marshal alone.
+	UseCachedSize bool
+
 	pragma.NoUnkeyedLiterals
 }
 
@@ -93,6 +113,7 @@ func (o MarshalOptions) marshalMessageFast(b []byte, m Message) ([]byte, error)
 			copy(x, b)
 			b = x
 		}
+		o.UseCachedSize = true
 	}
 	return methods.MarshalAppend(b, m, protoiface.MarshalOptions(o))
 }
diff --git a/proto/size.go b/proto/size.go
index a817cf7f..ab21cb34 100644
--- a/proto/size.go
+++ b/proto/size.go
@@ -13,6 +13,11 @@ import (
 
 // Size returns the size in bytes of the wire-format encoding of m.
 func Size(m Message) int {
+	return MarshalOptions{}.Size(m)
+}
+
+// Size returns the size in bytes of the wire-format encoding of m.
+func (o MarshalOptions) Size(m Message) int {
 	if size, err := sizeMessageFast(m); err == nil {
 		return size
 	}
diff --git a/runtime/protoiface/methods.go b/runtime/protoiface/methods.go
index e7d92db2..42832de0 100644
--- a/runtime/protoiface/methods.go
+++ b/runtime/protoiface/methods.go
@@ -27,10 +27,6 @@ type Methods struct {
 	// Size returns the size in bytes of the wire-format encoding of m.
 	Size func(m protoreflect.ProtoMessage) int
 
-	// CachedSize returns the result of the last call to Size.
-	// It must not be called if the message has been changed since the last call to Size.
-	CachedSize func(m protoreflect.ProtoMessage) int
-
 	// Unmarshal parses the wire-format message in b and places the result in m.
 	// It does not reset m or perform required field checks.
 	Unmarshal func(b []byte, m protoreflect.ProtoMessage, opts UnmarshalOptions) error
@@ -55,6 +51,7 @@ const (
 type MarshalOptions struct {
 	AllowPartial  bool
 	Deterministic bool
+	UseCachedSize bool
 
 	pragma.NoUnkeyedLiterals
 }