2 ways that i look at your query.
1. If the record length's vary, because of column width, and the flat file is a fixed width one,
then set the Max. of the column width in the sequential stage -> Properties -> Outputs -> Columns -> length.
2. If there is a delimiter, then update the columns field according to the text file columns, and set the delimiter under sequential stage -> Properties -> Outputs -> format -> delimiter with the delimiter in the text file. In case it is tab delimited, then put in 009 in the delimiter text box.
Note here : See that the fixed width checkbox is not checked.
I hope this helps, otherwise, post in your exact question along with example.
Cheers.