Both fieldFixedLengths and fieldNames parameters should be proportional in the content conversion.
Example:
Let us assume we have two fields (fielda, fieldb) then specify their size or length... as below
node.fieldFixedLengths: 10,20
node.fieldNames: fielda,fieldb
Note: Basically you specify fielda size is 10 and fieldb size is 20
Refer this link too