When you refer to the Dirty Birdy 20 what particular model or plan are you referring to?
The reason I ask is because Bridi never kitted a Dirty Birdy 20, what he did kit was a model called the Tweedy Bird (TB). This last had a 48-3/4" wing span with 400 squares.
Both David Wood and myself scaled Dirty Birdy 60 plans to produce a "20 size" version. David's were scaled to 48" if memory serves, mine were scaled to the same span as the TB which produced a wing with slightly more area; ~408 squares, again, if memory serves.
You might be interested in the following thread:
Recent posts talk about the plans.