I have found my dream trailer. It has a GVWR if 19000+ pounds. Owner says she pulls it just fine with her F350 dually. The trailer doesn't seem any bigger than those that pull into every barrel race behind regular pickups. Who knows about these things? Advice please.

You need to look up the gross combined vehicle rating on your truck. My 2019 F350 dually has a 31,000  combined rating. I pull my LQ that's rated for 16,000. Truck is rated 14,400. So I am just under the combined. If I were pulling  your trailer I would get a F450. Also, it's FEDERAL law that pulling any combined truck / trailer over 26,000 you need a commercial drivers license. It's federal , regardless of what your state is. Friend of mine was in an accident and insurance would not pay because they were " under licensed " they had to pay out of pocket and lost everything. It does not matter if your not actually hauling commercial , it's based on weight and weight Alone. I just got my CDL few months ago so I could sleep at night. In CA they are pulling people over and impounding your rig and horses until a CDL driver can come get you. 

Are they pulling people over just to make sure they aren't over the 26000? Or are they stopped for another reason and then check the weight?

Better to find out what the trailer actually weighs. The GVWR on my trailer is around 20k but I have the original weight sticker from the manufacturer and it weighs 15klbs. It's a 4 horse, 35 foot on the floor but it's a 99 so it's built well and heaaavvyy. I do pull it just fine with my tuned up Chevy 3500 though I wouldn't go any heavier. 

They pull them over for any stupid excuse. Speeding, etc. then they ask for the class A. Your sunk if you don't have it. And it does NOT matter what your ACTUAL weight is, weight slips don't mean squat. You have to be licensed to what your door sticker on truck and weight plate on trailer say you " can" pull. 

Not arguing but a Class A CDL is for a combination rig with air brakes. There are other classes of licensure for horse rigs that exceed weight and/or length that exceed what a regular drivers license covers. Not sure if it's Class B or a special class, but I am absolutely positive that a Class A is all about the combination rig with air brakes (I have Class A). 

OP - it's always been my understanding that it's not what your truck can haul but what it can stop safely. That being said it would depend on your normal drives. If you are on wholly flat ground it's easier to stop compared to hilly or mountainous driving. We hauled my 3 horse with 3 horses through the PA big hills (small mountains) with our 250 and had some rough moments. But I do fine now in my 4 horse trailer, even with 4 horses, in the smaller VA hills (very small mountains) with the same truck.

Nope not in CA.  Lol. All my written tests cover air brakes but a CDL class A is required for any combo of vehicles with a combination of 26001 and more  when I took my actual drive test for my CDL , I had to get an air brake exclusion on my license  anyway, no big deal  there is a FB page called " California class A drivers" that help us horse people get the license. RV endorsement does not work in CA . It's all very confusing and it might vary by state  I was in denial and didn't want to have to get it either but glad I did !  The irony about it all is the CA commercial class A handbook now has a picture of a dually wotu LQ trailer as its cover  Lolol  

I added a screen shot directly from CDL handbook 

It varies by state, in AR we are legal as either RV or farm tagged with no CDL. I can haul 80,000 pounds inside state lines and within 150 air miles of my farm shop with my regular license.   Federal law makes exceptions both for farm tags and RVs, it's up to the state how they interpret that. 

Very true! In CA you can do the same 150 mile thing but who would ever want to be limited to that lol. In CA they are total jerks about it because the fine is 2500.00 plus weight fees at a certain rate per pound your over. Friend of mine got a 9,000 dollar ticket !  We also have to go across the scales as well. 

In CA they are jerks about a lot!  You can't even have a flatbed on your pickup truck w/o it being deemed commercial!

Well sheesh!! California . .