I just bought my first ever pair of tall boots - Ariat Heritage Contour II Field Zip, size 10, full calf, medium height. Having never had tall boots before I went by the size chart on the website. They arrived yesterday. Foot size is perfect. They zip up snugly over the breeches I have. Nice. But they gouge the backs of my knees something awful. I realize as the ankles get broken in the height of the boots will drop some, but how much? Is this normal for new boots, to not be able to bend your knees at all? I mean, there’s no way I could ride in these things…not with my feet in the stirrups anyway. Will walking/hobbling around the house in them long enough soften the ankles to where they won’t interfere with the backs of my knees, or did I buy them too tall? I’m 5’10" and never in my life required anything “short.” Does anyone have any insight, experience, suggestions?
It’s possible they are just too tall for you. I have a pair that do this and have never got it to break in and go away. I think I might get a shoe repair person to cut down the back.
It depends on how stiff the boots are. I have heard anything between 1/4" to 1". If you think they are close, consider getting heal lifts to make up the hight difference until they drop. You may end up needing to always keep heal lifts in the boots
I have the same Ariat boots that I use for my everyday riding (but in tall height). Yes, mine rubbed into the back of my knees for the first day, riding two horses, but by the time I was done they’d broken down enough around the ankles so they don’t bother me anymore. That leather is actually really soft and pliable… almost to a fault. So you could just try suffering through a couple of rides and see if that helps.
However, as @Scribbler said, they could simply be too tall for you and your particular conformation. The way those boots are made, there isn’t really any room above the zipper to cut down, so if they’re that uncomfortable you may have to return/exchange them.
Pretty normal. Assuming they are leather the Tried and true method to help them breakin/ loosen up and drop is…really…put them on and turn the hose on them to wet throughly then ride and walk around as long as you can, like all day if possible. Also called the bathtub method, minus the tub. Allows the leather to relax and conform to the shape of your leg and foot. Usually works with just 1 or 2 sessions.
OBVIOUSLY you will condition after taking them off, insert boot trees or stuff with towels to hold their shape and air dry at room temp, not the trunk of your car in the sun or by a heater. That will ruin them.
If they are synthetic, no idea if that will help or not.
Use heel lifts.
I always buy tall boots that are way too tall. Every pair I’ve ever owned (from just about every manufacturer out there) drop significantly over time. With some, I’ve left the heel lifts in for a year or two, for others I pull them out within a week or two.
And yes, water is your friend. I always fill my boots with a hose and walk around (and ride) in them on day 1. Best, of course, to not be in the middle of winter at that point 
I am 5’8’ and have both the ariat heritage contours and the ariat kinsleys. Extra Tall for both. I firmly believe that the short term suffering is worth it for the look. Break in tips…get a spray bottlle with 3/4 water and 1/4 rubbing alcohol. Saturate the upper portion of the boots inside and outside just before you ride in them. I agree that heel lifts are lifesavers in the beginning. They will break in.
I have this problem because my feet are size 10, but I’m not tall. So all off-the-rack boots are too tall for me.
My barnmate shared this gem with me: https://www.smartpakequine.com/pt/equifit-heel-lifts-11082?utm_source=google&utm_medium=cpc&utm_content=NB_Shopping_Apparel-Gear_3P&utm_campaign=NB|3P+Apparel_Gear+>+Accessories&gclid=CjwKCAiAgbiQBhAHEiwAuQ6BkgDdFIUYKDSRVqzx-fWKZOFLaJZgvoyNC3BdvQYsIFcK6eoa3gRrfxoC_XgQAvD_BwE&gclsrc=aw.ds
This “lift” under your heel will cause the boot to hit you a little lower behind the knee. Totally solved my problem for $12!
(ETA: Oops, I realize someone else already gave you this suggestion! I didn’t read others’ comments.)
I used to work in a tack store and would fit boots all the time. For brand new boots (Ariat Heritages being the vast majority of what we sold), it’s super normal for them to stick into the back of your knee A LITTLE as they’re breaking in. Our rule of thumb for fitting height was 1/4" - 1/2" up your knee, so that when they drop they’re at the right height. If your boots are covering half your knee, they’re too tall. Heel lifts are really great if they’re just a smidge too tall
Beware of the calf - if your new boots were easy to zip up, they will be baggy in the calf when they’re broken in. You want the calf to be a little difficult to zip up (but still possible) when they’re brand new. Skin tight.
I also personally had great luck with the bathtub method if you think they’re the right height but need to be broken in 
Did this with my most recent pair of Ariats and they were fully broken in in a matter of days. And NO blisters!!!
Also for reference, I’m a size 11 medium height / regular calf in my Ariats
Thanks to everyone for the suggestions! I have ordered some heel lifts and will try the bathtub method, but this break-in process will have to be delayed for a couple months as winter in the midwest is not conducive to walking around in wet boots.
@marmarmar Thanks for the info. I think they are about 1/2" too tall right now so going by what you said they should eventually be just right. (fingers crossed). The calfs are definitely snug now and a bit difficult to zip, so I think they will turn out alright too.
