In the future, if you change the resolution in your scanner software, you can shrink the size of the scan (and therefore the file size) on the front end.
If you want to scan larger so that you can see more detail or archive the image or something, you can use a simple image editing program to change the image size (and therefore the file size) for uploading.
Irfanview is a free program that I use for quick, simple image cropping and resizing. It loads and runs super-fast and has the basic bells and whistles.
http://www.irfanview.com/
To resize an image, just open it up in irfanview, hit Control+R, change the image size, hit Enter, and then hit S to save the new smaller image. Easy peasy.