You can lower your file size by changing the resolution, but the setting that makes the biggest difference is your Color Mode setting. Most copiers today are able to scan in 3 color modes: Full Color, Grayscale, and Mono2. Grayscale and Color produce larger files and higher quality scans. Mono2 will make a file much smaller and allow more pages to be scanned in one job.
As an example, a machine in Full Color or Grayscale mode may only let a maximum of 5 pages go through; whereas the same machine may let 100 or more pages go through in Mono2 (depending on your machine and email server settings). Many copiers are also able to scan to USB where the size limits are less restrictive, non-existent, or limited only by the available space on your USB drive.