The image and file size attachment options are part of the Xenforo admin control panel, so moderators (like me) don't have any access to that. I'm an admin on another site that runs Xenforo and over there the maximum file size defaults to 16MB and there are no restrictions on pixel dimensions. Both of these can be set to whatever the site administration chooses, but I don't know what's being used here.
Will say that there's really no reason (that I can think of) why you would want to use an image over 4000 pixels wide. IIRC, Xenforo will automatically re-scale anything larger than about 1000 pixels wide (although you can view the full resolution version by clicking the image).
I don't think there's any (obvious) way to change the alert for images that are too large, that appears to be hard-wired into the forum software, but maybe there's a config file you can manually edit?
Personally, I'd agree that 800 pixels is a good size, maybe 1000 if there's some small details? I use photoshop to scale my images, but I'm sure there are other methods.... just got a new Mini with Sequoia and notice that if you right click an image, under quick actions there's a Convert option that has a choice of large/medium/small sizes for example. And Preview also has Tools > Adjust Size.
Also, if you use Apple Mail on iOS, when you attach a large photo it asks whether you want to reduce the size and gives several options. I often use that as a quick way to resize and e-mail the images to myself to post from my computer.