Brand new with a GIMP question

Hi. I am using GIMP 2.2 and trying to create an animated icon. I have down the principals and understand how they work. I'm just having one problem. No matter what I do I can't get the file size down to 40kb. I have tried resizing, scaleing and taking down the resoulution when I save the intital image. My qustion is: How do I tell what size file my .gif is going to be? Any other suggestions on how to reduce the file size? I have searched the tutorials and online and am having no luck. Any help would be much appreciated.