The right way to work on web UI junk is to press F12 to open the dev tools and adjust CSS. Once you figure out what HTML/CSS you need, then you can figure out what to change in WordPress. Web UI is nasty enough without WordPress between you and the HTML/CSS.
Find this DIV (a parent of the iframe):
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To find it, right click the Spine player, Inspect (probably called something else in Chrome, I use FF), then look at the parents. Hover the DIV in the web tools with the mouse so you can see the bounds in the page. Its bounds are not the size you want, so the iframe inside it is not the size you want. That DIV's parent has a flex layout:
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The flex layout doesn't know how tall you want its children.
You need a way to know how tall you want your player. You could make it a fixed height. More likely you probably want to use a CSS aspect ratio "hack" so that the player's height is based on its width, and then set a maximum width. We do this for example here (resize the page to see the first big player change width/height):
Blog: Spine 4.0 is here
Right click, Inspect, the first big player:
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The aspect ratio sizing works because reasons:
https://css-tricks.com/aspect-ratio-boxes/
If you find all this to be a huge mess, that's because it is. WordPress adds a TON of HTML junk. Suffering through this is just what people do. I suggest creating a minimal HTML page where you can work on your HTML/CSS without WordPress or any other junk. Get it working like you want there, at the size you want and behaving like you want when the page is resized (eg drag the edge of the dev tools to make the page more narrow), then try to transplant your working thing into WP.
Oh, one more thing, browsers have a limit for how many WebGL canvases they can have on a single page. I think 4 is safe, but you might want to try it on a few browsers (including mobile) or research it further. On our demos page we remove canvases that are off screen and reuse them for different demos as you scroll down. It's ridiculous and complex, but it works! You'd have to make your browser extremely tall to be able to tell that only 4 demos can be visible at once.
Spine: Demos
Actually, it looks like we use 5. You can see it if you zoom your browser waaaay out. The top 2 demos don't render because there's 7 demos visible and we use only 5 canvases.
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