diff --git a/README.md b/README.md
index c7de02a7..e39696a4 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,7 @@ You can provide an object of options as the last argument to `katex.render` and
 - `errorColor`: `string`. A color string given in the format `"#XXX"` or `"#XXXXXX"`. This option determines the color which unsupported commands are rendered in. (default: `#cc0000`)
 - `macros`: `object`. A collection of custom macros. Each macro is a property with a name like `\name` (written `"\\name"` in JavaScript) which maps to a string that describes the expansion of the macro.
 - `colorIsTextColor`: `boolean`. If `true`, `\color` will work like LaTeX's `\textcolor`, and take two arguments (e.g., `\color{blue}{hello}`), which restores the old behavior of KaTeX (pre-0.8.0). If `false` (the default), `\color` will work like LaTeX's `\color`, and take one argument (e.g., `\color{blue}hello`).  In both cases, `\textcolor` works as in LaTeX (e.g., `\textcolor{blue}{hello}`).
+- `maxSize`: `number`. If non-zero, all user-specified sizes, e.g. in `\rule{500em}{500em}`, will be capped to `maxSize` ems. Otherwise, users can make elements and spaces arbitrarily large (the default behavior).
 
 For example:
 
diff --git a/src/Options.js b/src/Options.js
index 55cdfd23..60dffc09 100644
--- a/src/Options.js
+++ b/src/Options.js
@@ -51,6 +51,7 @@ class Options {
         this.phantom = data.phantom;
         this.font = data.font;
         this.sizeMultiplier = sizeMultipliers[this.size - 1];
+        this.maxSize = data.maxSize;
         this._fontMetrics = null;
     }
 
@@ -66,6 +67,7 @@ class Options {
             color: this.color,
             phantom: this.phantom,
             font: this.font,
+            maxSize: this.maxSize,
         };
 
         for (const key in extension) {
diff --git a/src/Settings.js b/src/Settings.js
index 26a93ee2..efab9ce0 100644
--- a/src/Settings.js
+++ b/src/Settings.js
@@ -24,6 +24,7 @@ class Settings {
         this.errorColor = utils.deflt(options.errorColor, "#cc0000");
         this.macros = options.macros || {};
         this.colorIsTextColor = utils.deflt(options.colorIsTextColor, false);
+        this.maxSize = Math.max(0, utils.deflt(options.maxSize, Infinity));
     }
 }
 
diff --git a/src/buildTree.js b/src/buildTree.js
index aa3e24eb..773d326e 100644
--- a/src/buildTree.js
+++ b/src/buildTree.js
@@ -16,6 +16,7 @@ const buildTree = function(tree, expression, settings) {
     // Setup the default options
     const options = new Options({
         style: startStyle,
+        maxSize: settings.maxSize,
     });
 
     // `buildHTML` sometimes messes with the parse tree (like turning bins ->
diff --git a/src/units.js b/src/units.js
index f60550e9..0efdb0b6 100644
--- a/src/units.js
+++ b/src/units.js
@@ -90,7 +90,7 @@ const calculateSize = function(sizeValue, options) {
             scale *= unitOptions.sizeMultiplier / options.sizeMultiplier;
         }
     }
-    return sizeValue.number * scale;
+    return Math.min(sizeValue.number * scale, options.maxSize);
 };
 
 module.exports = {
diff --git a/test/katex-spec.js b/test/katex-spec.js
index 4a217db2..06e78f81 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -2308,3 +2308,25 @@ describe("Unicode", function() {
         expect("ΓΔΘΞΠΣΦΨΩ").toParse();
     });
 });
+
+describe("The maxSize setting", function() {
+    const rule = "\\rule{999em}{999em}";
+
+    it("should clamp size when set", function() {
+        const built = getBuilt(rule, new Settings({maxSize: 5}))[0];
+        expect(built.style.borderRightWidth).toEqual("5em");
+        expect(built.style.borderTopWidth).toEqual("5em");
+    });
+
+    it("should not clamp size when not set", function() {
+        const built = getBuilt(rule)[0];
+        expect(built.style.borderRightWidth).toEqual("999em");
+        expect(built.style.borderTopWidth).toEqual("999em");
+    });
+
+    it("should make zero-width rules if a negative maxSize is passed", function() {
+        const built = getBuilt(rule, new Settings({maxSize: -5}))[0];
+        expect(built.style.borderRightWidth).toEqual("0em");
+        expect(built.style.borderTopWidth).toEqual("0em");
+    });
+});