Add test

src/main/java/com/refinedmods/refinedstorage/api/util/IStackList.java

@@ -123,4 +123,9 @@ public interface IStackList<T> {
      */
     @Nonnull
     IStackList<T> copy();
+
+    /**
+     * @return the amount of elements in this list
+     */
+    int size();
 }

src/main/java/com/refinedmods/refinedstorage/apiimpl/util/FluidStackList.java

@@ -157,4 +157,9 @@ public class FluidStackList implements IStackList<FluidStack> {
 
         return list;
     }
+
+    @Override
+    public int size() {
+        return stacks.size();
+    }
 }

src/main/java/com/refinedmods/refinedstorage/apiimpl/util/ItemStackList.java

@@ -155,4 +155,9 @@ public class ItemStackList implements IStackList<ItemStack> {
 
         return list;
     }
+
+    @Override
+    public int size() {
+        return stacks.size();
+    }
 }