diff --git a/plugin/src/main/java/lol/pyr/znpcsplus/entity/EntityPropertyRegistryImpl.java b/plugin/src/main/java/lol/pyr/znpcsplus/entity/EntityPropertyRegistryImpl.java
index 821660f..b9d3e0a 100644
--- a/plugin/src/main/java/lol/pyr/znpcsplus/entity/EntityPropertyRegistryImpl.java
+++ b/plugin/src/main/java/lol/pyr/znpcsplus/entity/EntityPropertyRegistryImpl.java
@@ -149,8 +149,6 @@ public class EntityPropertyRegistryImpl implements EntityPropertyRegistry {
         // Phantom
         registerType("phantom_size", 0); // TODO
 
-        // Slime
-        registerType("slime_size", 0); // TODO
          */
     }
 
@@ -331,6 +329,16 @@ public class EntityPropertyRegistryImpl implements EntityPropertyRegistry {
             linkProperties("is_saddled", "is_eating", "is_rearing", "has_mouth_open");
         }
 
+        // Slime and Magma Cube
+        int sizeIndex;
+        if (ver.isNewerThanOrEquals(ServerVersion.V_1_17)) sizeIndex = 16;
+        else if (ver.isNewerThanOrEquals(ServerVersion.V_1_15)) sizeIndex = 15;
+        else if (ver.isNewerThanOrEquals(ServerVersion.V_1_14)) sizeIndex = 14;
+        else if (ver.isNewerThanOrEquals(ServerVersion.V_1_10)) sizeIndex = 12;
+        else if (ver.isNewerThanOrEquals(ServerVersion.V_1_9)) sizeIndex = 11;
+        else sizeIndex = 16;
+        register(new IntegerProperty("size", sizeIndex, 1, v1_8));
+
         if (!ver.isNewerThanOrEquals(ServerVersion.V_1_11)) return;
         // Spellcaster Illager
         int spellIndex = 12;
diff --git a/plugin/src/main/java/lol/pyr/znpcsplus/entity/properties/IntegerProperty.java b/plugin/src/main/java/lol/pyr/znpcsplus/entity/properties/IntegerProperty.java
index 2fe1c35..26f42c4 100644
--- a/plugin/src/main/java/lol/pyr/znpcsplus/entity/properties/IntegerProperty.java
+++ b/plugin/src/main/java/lol/pyr/znpcsplus/entity/properties/IntegerProperty.java
@@ -10,14 +10,22 @@ import java.util.Map;
 
 public class IntegerProperty extends EntityPropertyImpl<Integer> {
     private final int index;
+    private final boolean legacy;
 
-    protected IntegerProperty(String name, int index, Integer defaultValue) {
+    public IntegerProperty(String name, int index, Integer defaultValue) {
+        this(name, index, defaultValue, false);
+    }
+
+    public IntegerProperty(String name, int index, Integer defaultValue, boolean legacy) {
         super(name, defaultValue, Integer.class);
         this.index = index;
+        this.legacy = legacy;
     }
 
     @Override
     public void apply(Player player, PacketEntity entity, boolean isSpawned, Map<Integer, EntityData> properties) {
-        properties.put(index, newEntityData(index, EntityDataTypes.INT, entity.getProperty(this)));
+        properties.put(index, legacy ?
+                newEntityData(index, EntityDataTypes.BYTE, (byte) entity.getProperty(this).intValue()) :
+                newEntityData(index, EntityDataTypes.INT, entity.getProperty(this)));
     }
 }
diff --git a/plugin/src/main/java/lol/pyr/znpcsplus/npc/NpcTypeImpl.java b/plugin/src/main/java/lol/pyr/znpcsplus/npc/NpcTypeImpl.java
index de2afdf..5d72cbf 100644
--- a/plugin/src/main/java/lol/pyr/znpcsplus/npc/NpcTypeImpl.java
+++ b/plugin/src/main/java/lol/pyr/znpcsplus/npc/NpcTypeImpl.java
@@ -135,6 +135,9 @@ public class NpcTypeImpl implements NpcType {
             if (EntityTypes.isTypeInstanceOf(type, EntityTypes.ABSTRACT_PIGLIN)) {
                 addProperties("piglin_immune_to_zombification");
             }
+            if (EntityTypes.isTypeInstanceOf(type, EntityTypes.SLIME) || EntityTypes.isTypeInstanceOf(type, EntityTypes.PHANTOM)) {
+                addProperties("size");
+            }
             return new NpcTypeImpl(name, type, hologramOffset, new HashSet<>(allowedProperties), defaultProperties);
         }
     }