You can easily change the size of an image you have uploaded if it is too large. This can be done within the Media Manager.
Select Media in the control panel side navigation.
Upload your image, or find it in the Media Manager.
Select your image, and click Manage.
Select Edit in the dropdown
Here you can change the size of the image, and make any additional edits, including cropping, adjusting color and contrast, adding filters, etc.
Apply your changes and save. When you save, you can choose to either overwrite your image with the changes, or create a copy of it with a new name.