diff --git a/api/gc-embedder-api.h b/api/gc-embedder-api.h
index ad33bc170..30ba62946 100644
--- a/api/gc-embedder-api.h
+++ b/api/gc-embedder-api.h
@@ -66,6 +66,7 @@ GC_EMBEDDER_API inline struct gc_atomic_forward gc_atomic_forward_begin(struct g
 GC_EMBEDDER_API inline void gc_atomic_forward_acquire(struct gc_atomic_forward *);
 GC_EMBEDDER_API inline int gc_atomic_forward_retry_busy(struct gc_atomic_forward *);
 GC_EMBEDDER_API inline void gc_atomic_forward_abort(struct gc_atomic_forward *);
+GC_EMBEDDER_API inline size_t gc_atomic_forward_object_size(struct gc_atomic_forward *);
 GC_EMBEDDER_API inline void gc_atomic_forward_commit(struct gc_atomic_forward *,
                                                      struct gc_ref new_ref);
 GC_EMBEDDER_API inline uintptr_t gc_atomic_forward_address(struct gc_atomic_forward *);
diff --git a/benchmarks/simple-gc-embedder.h b/benchmarks/simple-gc-embedder.h
index 2c599655b..5f77a7cc9 100644
--- a/benchmarks/simple-gc-embedder.h
+++ b/benchmarks/simple-gc-embedder.h
@@ -168,6 +168,20 @@ gc_atomic_forward_abort(struct gc_atomic_forward *fwd) {
   fwd->state = GC_FORWARDING_STATE_ABORTED;
 }
 
+static inline size_t
+gc_atomic_forward_object_size(struct gc_atomic_forward *fwd) {
+  GC_ASSERT(fwd->state == GC_FORWARDING_STATE_ACQUIRED);
+  switch (tag_live_alloc_kind(fwd->data)) {
+#define SCAN_OBJECT(name, Name, NAME)                                   \
+    case ALLOC_KIND_##NAME:                                             \
+      return name##_size(gc_ref_heap_object(ref));                      \
+    FOR_EACH_HEAP_OBJECT_KIND(SCAN_OBJECT)
+#undef SCAN_OBJECT
+  default:
+    GC_CRASH();
+  }
+}
+
 static inline void
 gc_atomic_forward_commit(struct gc_atomic_forward *fwd, struct gc_ref new_ref) {
   GC_ASSERT(fwd->state == GC_FORWARDING_STATE_ACQUIRED);
diff --git a/doc/manual.md b/doc/manual.md
index 4156defdf..ab0656db0 100644
--- a/doc/manual.md
+++ b/doc/manual.md
@@ -140,7 +140,10 @@ acquired and completed the forwarding attempt.
 
 An `ACQUIRED` object can then be forwarded via
 `gc_atomic_forward_commit`, or the forwarding attempt can be aborted via
-`gc_atomic_forward_abort`.
+`gc_atomic_forward_abort`.  Also, when an object is acquired, the
+collector may call `gc_atomic_forward_object_size` to compute how many
+bytes to copy.  (The collector may choose instead to record object sizes
+in a different way.)
 
 All of these `gc_atomic_forward` functions are to be implemented by the
 embedder.  Some programs may allocate a dedicated forwarding word in all
diff --git a/src/whippet.c b/src/whippet.c
index 0b4dd61d6..af9b3dbd4 100644
--- a/src/whippet.c
+++ b/src/whippet.c
@@ -831,13 +831,6 @@ static inline size_t mark_space_object_size(struct mark_space *space,
   return granules * GRANULE_SIZE;
 }
 
-static inline size_t gc_object_allocation_size(struct gc_heap *heap,
-                                               struct gc_ref ref) {
-  if (GC_LIKELY(mark_space_contains(heap_mark_space(heap), ref)))
-    return mark_space_object_size(heap_mark_space(heap), ref);
-  return large_object_space_object_size(heap_large_object_space(heap), ref);
-}
-
 static int heap_has_multiple_mutators(struct gc_heap *heap) {
   return atomic_load_explicit(&heap->multithreaded, memory_order_relaxed);
 }