var uploadField = document.getElementById("file");
uploadField.onchange = function() {
if(this.files[0].size > 2097152){
alert("File is too big!");
this.value = "";
};
};
This example should work fine. I set it up for roughly 2MB, 1MB in Bytes is 1,048,576 so you can multiply it by the limit you need.
Here is the jsfiddle example for more clearence:
https://jsfiddle.net/7bjfr/808/