Should be right on the main screen
The other things mentioned would be Advanced Settings > Resolution and Quality/File Size
And then Filing Options > File Format > Image Only (This one will greatly reduce the size if available, but probably not a good idea as it makes the pdf unsearchable.)
In the case of keeping it searchable, while needing to further reduce size from the other things listed you could do so after the fact.
For instance, if you have Adobe Acrobat (not reader) you can go to File > Save as other... and choose Reduced Size PDF , or Optimized PDF...
Lastly there is an oddity in Xerox scans where choosing Auto instead of Color or BW gives a higher file size. So set Output Color to Either Color or BW, not Auto (This won't make a big difference, but every Kb helps)