PBI Rookie:
The interface on my computer is a bit different than yours, but try this: at the top of the Steel Joist Settings panel is the parameter "Type of Steel Joist". Change this from "K" to "Custom", then you should be able to set the length to any value.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David