Hi Weeg. I think you may have also PM'ed me but I'll reply here for the potential benefit to other members!
We originally started with the Eglu Coop and attached run that was 3'x9'. I have 4 standard size chickens. Then I had a fence around a larger extended run. But my girls were jumping/flying over the 5' high fence. Plus we have hawks and coyotes here so I knew we needed a run with a top. So we got the Omlet Walk In Run...which we absolutely love!
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My girls don't currently get to free range because I have a dog. So we got the largest configuration they have online. Which is 9' wide by 24' long. And 6'7" tall. It's worked out awesome! It is very secure and sturdy. No problem to add a cover and it will have l no issue supporting snow. The roof is angled so it will also melt and drain off nicely. We are in Southern California so don't get much precipitation fortunately (well unfortunate for the water supply and environment).
I'm in the process of adding flooring since my grass has now all been eaten and destroyed. Will likely use wood chips and pine bedding per suggestions from other members. I'll probably add and 'edging' around the run so it doesn't get kicked out all over the place.
In terms of assembly...parts came in 21 boxes! But with great instructions. Think a big piece of IKEA furniture! I assembled it myself! I'm handy but no construction worker so I'd say it's easy. It took me probably 10-12 hours of hands-on time over 2 days. Mine was a non-standard assembly since I was working up against the house (with different equipment like electrical panel, raceways, pipes, etc.) I was working around and also in the back I wanted it tucked under the trees so did some customization to it.
I don't know anything about ducks but I'd say purchase the largest you can manage, even if they get to free range. The product is not cheap but a great investment you'll use forever (if you wanted to!).
Gosh I can't even think what I don't like about it...
Anyhow this is a very long reply so I'll stop there for now. But let me know if you want to know anything else. We love their products and I'm happy to share if it's helpful!