diff --git a/js/test_cases/helpers_size/README.md b/js/test_cases/helpers_size/README.md
new file mode 100644
index 0000000..d39036d
--- /dev/null
+++ b/js/test_cases/helpers_size/README.md
@@ -0,0 +1,9 @@
+Excerpt from [the DustJS tutorial](https://github.com/linkedin/dustjs/wiki/Dust-Tutorial#size_keyxxx___size_helper_Available_in_Dust_V11_release):
+
+    Array - number of elements, [1,2,3,4] has size=4
+    String - length of the string, "abcdef" has size=6
+    Object - Number of properties in the object, {a:4, b:8, c:15, d:16} has size=4
+    Number - Value of the number, 23 has size 23 and 3.14 has size 3.14
+    Undefined, 0, empty string - zero
+    Any other value - length after conversion to string
+
diff --git a/js/test_cases/helpers_size/inputarray.json b/js/test_cases/helpers_size/inputarray.json
new file mode 100644
index 0000000..98f6406
--- /dev/null
+++ b/js/test_cases/helpers_size/inputarray.json
@@ -0,0 +1,7 @@
+{
+  "val": [
+    "Alice",
+    "Bob",
+    "Chris"
+  ]
+}
diff --git a/js/test_cases/helpers_size/inputemptystring.json b/js/test_cases/helpers_size/inputemptystring.json
new file mode 100644
index 0000000..f911fb5
--- /dev/null
+++ b/js/test_cases/helpers_size/inputemptystring.json
@@ -0,0 +1,3 @@
+{
+  "val": ""
+}
diff --git a/js/test_cases/helpers_size/inputfalse.json b/js/test_cases/helpers_size/inputfalse.json
new file mode 100644
index 0000000..9cc7fda
--- /dev/null
+++ b/js/test_cases/helpers_size/inputfalse.json
@@ -0,0 +1,3 @@
+{
+  "val": false
+}
diff --git a/js/test_cases/helpers_size/inputmissing.json b/js/test_cases/helpers_size/inputmissing.json
new file mode 100644
index 0000000..95cf907
--- /dev/null
+++ b/js/test_cases/helpers_size/inputmissing.json
@@ -0,0 +1,3 @@
+{
+  "notval": "Alice"
+}
diff --git a/js/test_cases/helpers_size/inputnumber.json b/js/test_cases/helpers_size/inputnumber.json
new file mode 100644
index 0000000..378b4a9
--- /dev/null
+++ b/js/test_cases/helpers_size/inputnumber.json
@@ -0,0 +1,3 @@
+{
+  "val": 7.21
+}
diff --git a/js/test_cases/helpers_size/inputobject.json b/js/test_cases/helpers_size/inputobject.json
new file mode 100644
index 0000000..4ead7ce
--- /dev/null
+++ b/js/test_cases/helpers_size/inputobject.json
@@ -0,0 +1,6 @@
+{
+  "val": {
+    "pet": "cat",
+    "name": "fluffy"
+  }
+}
diff --git a/js/test_cases/helpers_size/inputstring.json b/js/test_cases/helpers_size/inputstring.json
new file mode 100644
index 0000000..e462e67
--- /dev/null
+++ b/js/test_cases/helpers_size/inputstring.json
@@ -0,0 +1,3 @@
+{
+  "val": "Alice"
+}
diff --git a/js/test_cases/helpers_size/inputtrue.json b/js/test_cases/helpers_size/inputtrue.json
new file mode 100644
index 0000000..25d6b1f
--- /dev/null
+++ b/js/test_cases/helpers_size/inputtrue.json
@@ -0,0 +1,3 @@
+{
+  "val": true
+}
diff --git a/js/test_cases/helpers_size/inputzero.json b/js/test_cases/helpers_size/inputzero.json
new file mode 100644
index 0000000..d6110e8
--- /dev/null
+++ b/js/test_cases/helpers_size/inputzero.json
@@ -0,0 +1,3 @@
+{
+  "val": 0
+}
diff --git a/js/test_cases/helpers_size/main.dust b/js/test_cases/helpers_size/main.dust
new file mode 100644
index 0000000..fab1f99
--- /dev/null
+++ b/js/test_cases/helpers_size/main.dust
@@ -0,0 +1,3 @@
+The size of val ({val|js|s}) is {@size key=val /}{~n}
+The size of "{val}" is {@size key="{val}" /}{~n}
+The size with no key is {@size /}{~n}
diff --git a/src/bin.rs b/src/bin.rs
index 3c4c8c2..0f279c7 100644
--- a/src/bin.rs
+++ b/src/bin.rs
@@ -15,6 +15,7 @@ use renderer::Loopable;
 use renderer::MathNumber;
 use renderer::RenderError;
 use renderer::Renderable;
+use renderer::Sizable;
 use renderer::Truthiness;
 use renderer::WalkError;
 use renderer::Walkable;
@@ -313,6 +314,12 @@ impl Loopable for serde_json::Value {
     }
 }
 
+impl Sizable for serde_json::Value {
+    fn get_size<'a>(&'a self) -> Option<IceResult<'a>> {
+        todo!()
+    }
+}
+
 impl Castable for serde_json::Value {
     fn cast_to_type<'a>(&'a self, target: &str) -> Option<IceResult<'a>> {
         match (self, target) {
diff --git a/src/renderer/context_element.rs b/src/renderer/context_element.rs
index 56d194c..01281d3 100644
--- a/src/renderer/context_element.rs
+++ b/src/renderer/context_element.rs
@@ -22,6 +22,7 @@ pub trait ContextElement:
     + FromContextElement
     + IntoRcIce
     + Castable
+    + Sizable
 {
 }
 
@@ -63,6 +64,10 @@ pub trait Castable {
     fn cast_to_type<'a>(&'a self, target: &str) -> Option<IceResult<'a>>;
 }
 
+pub trait Sizable {
+    fn get_size<'a>(&'a self) -> Option<IceResult<'a>>;
+}
+
 pub trait CastToAny {
     fn to_any(&self) -> &dyn Any;
 }
diff --git a/src/renderer/mod.rs b/src/renderer/mod.rs
index 571a02b..9b1f5b7 100644
--- a/src/renderer/mod.rs
+++ b/src/renderer/mod.rs
@@ -18,6 +18,7 @@ pub use context_element::IceResult;
 pub use context_element::IntoContextElement;
 pub use context_element::Loopable;
 pub use context_element::Renderable;
+pub use context_element::Sizable;
 pub use context_element::Truthiness;
 pub use context_element::Walkable;
 pub use errors::CompileError;
diff --git a/src/renderer/parameters_context.rs b/src/renderer/parameters_context.rs
index a886586..0d45fbc 100644
--- a/src/renderer/parameters_context.rs
+++ b/src/renderer/parameters_context.rs
@@ -14,6 +14,7 @@ use crate::renderer::DustRenderer;
 use crate::renderer::Loopable;
 use crate::renderer::RenderError;
 use crate::renderer::Renderable;
+use crate::renderer::Sizable;
 use crate::renderer::Truthiness;
 use crate::renderer::WalkError;
 use crate::renderer::Walkable;
@@ -189,6 +190,12 @@ impl Walkable for OwnedLiteral {
     }
 }
 
+impl Sizable for OwnedLiteral {
+    fn get_size<'a>(&'a self) -> Option<IceResult<'a>> {
+        todo!()
+    }
+}
+
 impl Castable for OwnedLiteral {
     fn cast_to_type<'a>(&'a self, target: &str) -> Option<IceResult<'a>> {
         match (self, target) {