New paddler, mostly flatwater, some gentle current. Looking at Dagger Zydeco, uncertain if he needs 11 or 9?

“Needs”?? You might fit in both – depends on your inseam and weight. But that does not mean the 9 would be a good choice. Note that maximum weight for the 9 is 220 lbs. so if you are over 175 pounds it’s probably not the best choice – being close to the capacity of a boat makes it sit lower in the water, which affects performance. The rating is for your body weight PLUS your clothing and gear and anything you are hauling. Even if you are under 175 pounds a 9’ boat is awfully small for an average sized guy and it won’t be very versatile.

The 11, with a 325 lb rating, is a little narrower and with a longer waterline will track better and go faster with less effort. The 9 also has no hatches and I don’t think it has dual bulkheads (which the 11 does). You would need at least one flotation bag with the 9.

As @willowleaf states, paddler height has very little to do with recommended boat length. The 9’ long, 28.5" wide Zydeco will be slow and won’t track well. The 11’ model will paddle better but it’s still has no front bulkhead and no deck perimeter lines. If he’s sure that all he ever wants to do is paddle flat water and not learn true paddling skills then the Zydeco will be fine, but otherwise I would suggest something more like an Alchemy.

I stand corrected – the Zydeco 11 only has one bulkhead (which means if swamped or capsized it will tend to sink nose down) which means you will need an inflatable flotation bag for the bow cavity.

In general, kayaks (other than for whitewater) under 12’ are strictly for “lily-dipping” (slow paddling or floating in sheltered flatwater) or for smaller people or children. Should not be used in coastal waters or wide lakes because currents and wind can push them far beyond where it would be practical to get back to shore. And, because they are so wide, they tend to be unstable in rough water and waves. Longer boats don’t necessarily weigh more because they can be narrower without losing volume.

There are some higher end exceptions: Eddyline makes a very cool 10’ kayak called the Sky for $1000 with a 250 pound capacity that is only 26" wide. Being thermoformed, it weighs a whopping 16 pounds less than the Zydeco 11.