Id like to know everyone’s views on this topic.
In a DW setting, assumes 22kts and using the same foil (1200 width) with a paddle or stowed Parawing, what is easier to keep on foil : a DW board like 8ft x 19", or a mid length say 6’3" x 21?
Does the longer board provide better pitch authority? Or is the shorter board best because of weight ?
Keen to hear opinions.
I always thought that while flying, the less board the better.
IMHO and experience - it’ll mostly depend on how well balanced each board distributes it’s volume - this will affect swing weight on pitch.
But saying both boards are trully well balanced and mast is in optimal balanced position with the preffered front/back pressure (this will be adjusted according to each foil-fuse-stab combo) - then longer board generates a little bit more pitch swings, you have more nose more tail generating a different swing. Shorter board should be easier to pump always, but I don’t think it’s a super big deal breaker.
This aspect with some other factors are why in my home spots conditions I don’t paddle so much, I get steady winds so can trust going out for longer DW on 2 parawings and enjoy way more being on the 5’5"x18.5"x55L parawing board hands free downwind rather than the 7’6"x17,75"x92L with the paddle. But still enjoy taking the paddle out every once in a while to keep things spiced and variable. And not lose the SUP skill 
Once you are on foil the smaller the board the better. That goes for all foil disciplines.
going back and fourth between dw SUPing a 7’6" x 19" 95L and 5’10" x 19" 95L the difference in pump is negligible, better yes but not by much worth noting. IMO on a good light weight board even the swing weight is fairly negligible. What you really gain is lack of wind pushing your board around on the smaller one, that is very noticeable as well as touch points. you can crank turns way harder without worrying about the board touching down. it’s definitely a better surfier feel on the smaller board and there’s just a better feel in general with riding a smaller board. There may be more of an argument for boards in the 8’+ range but I don’t have any need for boards that big anyway so personally would never really ride them. 22kts is pretty good conditions, foil selection has a lot more to do with what’s easier to keep on foil. example 22kts in the ocean here in jersey at 180lbs you can ride the code 810x or 890x and maybe work a little but it’s going to be fun, 980s you can stand there and barely burn a calorie.
another debate, would be feels with length vs width and 21" wide for downwind doesn’t sound very fun to ride. to me an 8’x19" board feels smaller under my feet than a 6’3" that’s 21" wide. think something mid 6’ and ideally 19-20" wide max would be much better feeling though still fairly wide
On that foil, marginal difference, likely imperceptible or confused by other more obvious factors, at a beginner level even more so would be my 2c
For me it’s ML for sure, perhaps a prone board would be even better but I’ve never DW’d a prone board.
DW board is harder to pump and easier to slap your tail
ML board is a bit harder to pump but really good at recovering touchdowns
Dock board is easiest to pump but lack of ability to recover touchdowns really sucks