Hi Drew,
You have to apply the correct page that is used as your lookup.
For the Item Lookup, use the following:
pageextension 50000 MyExtension extends "Item Lookup"
{
layout
{
modify("No.") // column you want to modify
{
Width = 50; // enough space for 50 characters
}
}
}
Works a treat for me:

And yes, auto-sizing of the column width does have an effect only when you resize the browser. So, it doesn't work in all situations, especially lookups.