There isn't a separate font code for the Sku number on the tag.
However, there is an adjustment option that has helped many people who want the Sku number to print larger.
From Preferences add the key word SKURETAILFONT anywhere in the Miscellaneous Options box
This will make the Sku number print with the same font as the retail price.
I.e., if you make the price larger by setting a retail font code, you can make the Sku print larger too.
The exact code that you would use for the Retail Font Code would depend on what language your printer uses and what size you want.
For example, for printers using the Datamax language (Printer type D), a common large size font code is 1211000
Another example, for printers using the Zebra language (Printer type Z), a common large size font code is C or D or E