diff --git a/runtime/src/main/java/org/capnproto/BuilderArena.java b/runtime/src/main/java/org/capnproto/BuilderArena.java
index bcd5cf5..cb0da8a 100644
--- a/runtime/src/main/java/org/capnproto/BuilderArena.java
+++ b/runtime/src/main/java/org/capnproto/BuilderArena.java
@@ -6,12 +6,28 @@ import java.util.Vector;
 
 public final class BuilderArena implements Arena {
 
+    public enum AllocationStrategy {
+        FIXED_SIZE,
+        GROW_HEURISTICALLY
+    }
+
+    public static final int SUGGESTED_FIRST_SEGMENT_WORDS = 1024;
+    public static final AllocationStrategy SUGGESTED_ALLOCATION_STRATEGY =
+        AllocationStrategy.GROW_HEURISTICALLY;
+
     // Maybe this should be ArrayList?
     public final Vector<SegmentBuilder> segments;
 
-    public BuilderArena() {
+    public int nextSize;
+    public final AllocationStrategy allocationStrategy;
+
+
+    public BuilderArena(int firstSegmentSizeWords, AllocationStrategy allocationStrategy) {
         this.segments = new Vector<SegmentBuilder>();
-        SegmentBuilder segment0 = new SegmentBuilder(ByteBuffer.allocate(1024 * 8));
+        this.nextSize = firstSegmentSizeWords;
+        this.allocationStrategy = allocationStrategy;
+        SegmentBuilder segment0 = new SegmentBuilder(
+            ByteBuffer.allocate(firstSegmentSizeWords * Constants.BYTES_PER_WORD));
         segment0.buffer.mark();
         segment0.buffer.order(ByteOrder.LITTLE_ENDIAN);
         this.segments.add(segment0);
diff --git a/runtime/src/main/java/org/capnproto/MessageBuilder.java b/runtime/src/main/java/org/capnproto/MessageBuilder.java
index fa9f172..4f4f3bd 100644
--- a/runtime/src/main/java/org/capnproto/MessageBuilder.java
+++ b/runtime/src/main/java/org/capnproto/MessageBuilder.java
@@ -5,7 +5,18 @@ public final class MessageBuilder {
     private final BuilderArena arena;
 
     public MessageBuilder() {
-        this.arena = new BuilderArena();
+        this.arena = new BuilderArena(BuilderArena.SUGGESTED_FIRST_SEGMENT_WORDS,
+                                      BuilderArena.SUGGESTED_ALLOCATION_STRATEGY);
+    }
+
+    public MessageBuilder(int firstSegmentWords) {
+        this.arena = new BuilderArena(firstSegmentWords,
+                                      BuilderArena.SUGGESTED_ALLOCATION_STRATEGY);
+    }
+
+    public MessageBuilder(int firstSegmentWords, BuilderArena.AllocationStrategy allocationStrategy) {
+        this.arena = new BuilderArena(firstSegmentWords,
+                                      allocationStrategy);
     }
 
     public <T> T getRoot(FromStructBuilder<T> factory) {