Segment or Filtering on character amount

I am not aware of any sort of "length" condition in the segment builder, but you could potentially use Classifications on your dimension, and use regex rules to set up a "length of sku" classification.

You could then create a segment based on the classification length.

Regex for length of 6:
^(.){6}$

Regex for length of 7:

^(.){7}$

(basically, look for any character except line breaks, with the specified length {x}, and start from the beginning of the string to the end of the string (so that {6} doesn't match both the 6 and 7 lines.

Classifications will process every 4-6 hours, so this won't work if you need the data immediately, but if you are reporting based on yesterday or earlier, you should be fine... Classifications will also apply backwards up to 6 months.... so this should cover a lot of your needs (you just need to wait about 24 hours for the initial rule setup to finish processing upon setting this up)

If the SKU is an integer datatype, you could try using Greater Than / Less Than functions.  If Greater Than 999999 = 7 digit SKUs.  Less Than 1000000 = 6 digit SKUs.