I’d like to know this as well. From what I saw on the verge review, XS Max has the same amount of content on the screen as the X. So I would assume it will fit fine if the app has been updated for the X
I wonder if it will be better than the Plus models were. I haven't had a Plus since the 7, but my experience was that all of the apps were the same as the smaller phones in portrait. Only landscape on some apps was different. It was one of my biggest complaints about the Plus. All the penalties of the size without the benefit of apps making use of the space in portrait. I tried to like landscape, but the phone wasn't tall (in landscape) enough to really be usable when the keyboard was out.
Based on video reviews some apps will have graphical glitches before they are updated to support the XS Max. Others will work just fine but will not make any use of the extra screen space of the XS Max.
I doubt most apps will make any good use of the larger screen beyond any landscape mode tweaks.
He mentions that on the Max some apps have UI glitches that don't necessarily cause a major issue, but some elements of the UI can end up being cut off, he mentions Instagram that you can't see the caption you're typing on a post due to it being cut off.
I think the adoption of the screen size will be much faster than in previous years.