For Example -

$('input[name="attachment"]').change(function(){
var fileInput = document.getElementById('test_id');
var files = fileInput.files;
/ checking for greater than or equals to 2MB file size /
for (let i = 0; i < files.length; i++) {
var fileSize = files[i].size;
/ 1024 = 1MB /
var size = Math.round((fileSize / 1024));
if (size > 2*1024) {
alert("Invalid file size, please select a file less than or equal to 2mb size");
fileInput.value = '';
return false;
}
}

}
if(files.length==0){
alert("Please choose a file first...");
return false;
}

})