In this tab u can adjust the wordsize of the username and password. The number varies acording to the billingcompagny of the site u wish to ......ehm..test :-)=
For example, ccbill has for both username and pasword the wordlength of mininmal 6 and maximum 8 digits.
Ibill (as seen in the picture) has the sizes min 3 and max 15. These r standard amounts, u must test and c if a specific site has more or less digits, but thats a thing of practice. For a list of wordsize by billingcomp look here .