I could be wrong on this, but I think the only way to get it so you can specify a custom length when placing is to create an iPart table and set the length parameter (B_L did you say?) as a custom column. Does that make sense?
Even if you only have one configuration of the part (with varying length, of course), you can set up an iPart table with only one row. Then you would add the length parameter as a column, right-click that column and set it to be custom. Take a look at the example part I posted earlier. The PL column in the iPart table is set up as a custom column.
I'm not sure this explanation makes much sense, so post back if I wasn't clear enough.
Simply make the LENGTH property a key property in the part family.
This will give you a length dialog box when inserting the part.