Remember that costco has a great return policy for most things. And you're not paying a lot for the boots anyway. If nothing else you could wear them around town if they don't work for hiking.

I know shoes or boots fit by feel, which is a skill you develop after a while. You kind of know if they'll work for you once you've figured out a shoe that works. I had a tough time finding a good fit - heel to toe blisters on my first pair of shoes after 8 miles or so of backpacking.... The boots should have plenty of room in the toe box, and the heel shouldn't pop out of the boot as you walk/hike. It shouldn't rub or chafe the sides of the feet, bang the toes or require you to tape up or slap on moleskin to use them.

I'd dayhike with them with a full pack and see how it goes.
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"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

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