chrome.app.runtime.onLaunched.addListener(function() {
    chrome.app.window.create('html/beagle.html', {
        width: 1024,
        height: 768
    }, function(win) {
        win.onClosed.addListener(function() {
            console.log("called at onClosed");
            serial_lib.closeSerial(function() {
                console.log("close successfully");
            });
        });
    });
})

@@ -99,16 +99,40 @@
 tml', %7B%0A
+        'bounds': %7B%0A    
         
@@ -152,16 +152,20 @@
         
+    
 height: 
@@ -168,16 +168,26 @@
 ht: 768%0A
+        %7D%0A
     %7D, f

Use bounds property to set window size