Timely thread, as I'm just beginning to deal with this.
To throw in an additional complicating thought, on some products I'll be dealing with, different sizes or lengths will likely affect shipping costs substantially.
Am also not familiar enough with their system yet, but I think that some sizes and dimensions are built into traditional product numbers (beyond SKUs), that involve the entire distribution network, printed brochures, etc.
Some sizes, I know, also affect product popularity, by an overwhelming amount, so much that one would want to link to this different size separately. :(