H&M sizes vs. Zara

Hello everyone, just want to say first off that I recently discovered this website and it has helped me immensely in crafting my wardrobe. I live in Kansas, (a.k.a the middle of the U.S), and there is not much fashion wise to choose from here. We do have an H&M which I frequent a lot and I am a small in most shirts, t-shirts, and coats/jackets. I also recently discovered Zara had online ordering and they have some awesome shirts so I went ahead and ordered a small t-shirt. I'm starting to think that was a dumb decision because after reading about them they are notorious for smaller sizes but I assumed since I'm a small at H&M I would be fine. I was wondering if anyone knew size comparisons with these places because now I'm worried it will be too small. Any advice appreciated. Thanks!

I've always found the H&M sizes to be completely inconsistent, but that said I'm usually a medium. Zara's sizing, like you said does come up smaller and I often take a large in their clothing.