How do I change the size of it?

size as in dimensions or size as in the kb's?
if its the latter, you can just reduce the quality, save as jpg isntead of png.

Image Size: Use photoshop, paint.net, croppola, or any image editing service to reduce the size.
Filesize: If it is saved as .png, google for PNG Optimizer. If not, you can reduce the quality when saving. Normally it can be reduced relatively much before it becomes starkly visible.