The length attribute of a variable is the number of bytes used to store each of its values in a SAS data set. The default length of variables in this context is 8.
You should have no need to use a length other than 8 for numeric variables, although values between 3 and 8 are acceptable. For character variables, use the length of the longest value to be stored in the variable, up to a maximum of 200. Note that values of length greater than 16 may be truncated in some parts of SAS/LAB software.