diff --git a/bootbox.js b/bootbox.js
index cbefb471..2adbfbf4 100644
--- a/bootbox.js
+++ b/bootbox.js
@@ -572,6 +572,7 @@
     options = sanitize(options);
 
     var dialog = $(templates.dialog);
+    var innerDialog = dialog.find(".modal-dialog");
     var body = dialog.find(".modal-body");
     var buttons = options.buttons;
     var buttonStr = "";
@@ -598,6 +599,14 @@
       dialog.addClass(options.className);
     }
 
+    if (options.size === "large") {
+      innerDialog.addClass("modal-lg");
+    }
+
+    if (options.size === "small") {
+      innerDialog.addClass("modal-sm");
+    }
+
     if (options.title) {
       body.before(templates.header);
     }
diff --git a/tests/defaults.test.js b/tests/defaults.test.js
index 6fb671a5..9bffc76e 100644
--- a/tests/defaults.test.js
+++ b/tests/defaults.test.js
@@ -58,6 +58,41 @@ describe("bootbox.setDefaults", function() {
     });
   });
 
+  describe("size", function() {
+    describe("when set to large", function() {
+      beforeEach(function() {
+        bootbox.setDefaults({
+          size: "large"
+        });
+
+        this.dialog = bootbox.dialog({
+          message: "test"
+        });
+      });
+
+      it("adds the large class to the innerDialog", function() {
+        expect(this.dialog.children(":first").hasClass("modal-dialog")).to.be.true;
+        expect(this.dialog.children(":first").hasClass("modal-lg")).to.be.true;
+      });
+    });
+    describe("when set to small", function() {
+      beforeEach(function() {
+        bootbox.setDefaults({
+          size: "small"
+        });
+
+        this.dialog = bootbox.dialog({
+          message: "test"
+        });
+      });
+
+      it("adds the small class to the innerDialog", function() {
+        expect(this.dialog.children(":first").hasClass("modal-dialog")).to.be.true;
+        expect(this.dialog.children(":first").hasClass("modal-sm")).to.be.true;
+      });
+    });
+  });
+
   describe("backdrop", function() {
     describe("when set to false", function() {
       beforeEach(function() {
diff --git a/tests/dialog.test.coffee b/tests/dialog.test.coffee
index c65e6dfa..aa17e289 100644
--- a/tests/dialog.test.coffee
+++ b/tests/dialog.test.coffee
@@ -313,3 +313,22 @@ describe "bootbox.dialog", ->
 
         it "should not hide the modal", ->
           expect(@hidden).not.to.have.been.called
+
+  describe "with size option", ->
+    describe "when the size option is set to large", ->
+      beforeEach ->
+        @dialog = bootbox.dialog
+          message: "test"
+          size: "large"
+                                                            
+      it "adds the large class to the innerDialog", ->
+        expect(@dialog.children(":first").hasClass("modal-lg")).to.be.true
+
+    describe "when the size option is set to small", ->
+      beforeEach ->
+        @dialog = bootbox.dialog
+          message: "test"
+          size: "small"
+                                                            
+      it "adds the large class to the innerDialog", ->
+        expect(@dialog.children(":first").hasClass("modal-sm")).to.be.true