Generally, it's size 6. However, Alden definitely makes smaller sizes. I've seen them pop up on eBay and in Asia from time to time. You can email email@example.com or reach out to TheShoeMart, Alden DC, or Alden SF.
Tan laces would look cool there, especially with the contrast stitching. Nice boots.
I am fairly certain 6D is the smallest that Alden will make at this point. Alden used to make women and even boy shoes in the distant past. But that is history now. However, it doesn't hurt to contact Alden if you need smaller than that.