At this time, there are no Attributes for the ROWSET var. Only the FILELIST var.
To determine max LRECL for a flatFile .. you would have to read from Rec1 through RecX and snapshot the row length.
Depending on your installed system, you might could run a system command (wc -L/linux) and pull the value you need back into the project that way with a quicker turn-around.
I'm sure there are options in Windows, etc as well.