Just how do Macros work in Craft 3?
To answer your title question... macros in Craft 3 work exactly the same as they do in Craft 2. :)

The problem you are hitting is here:

{{ macros sizedImage...


Seems like a straightforward typo. I'm assuming you meant to write it like this:

{{ macros.sizedImage...


For that reason (among others), I would recommend not naming your imported macros as "macros". It just invites confusion, and leads to silly typos like this one.

Personally, I often name them "render" (though you can truly name them anything) because it's more obvious what's happening.

{% import 'macros/global' as render %}
{{ render.sizedImage... }}