Sorry about the late reply, been pretty busy of late. There are a couple of reasons for the minimum resolution requirement. For one; vectors are commonly most useful as resources for extended projects such as wallpapers, videos, etc. Large resolutions allow makers to easily utilize all or part of the vector in an image. (They may want the face to take up the whole screen or something like that.) As far as why its a requirement rather than an option its a bit of a validation thing. It's much easier to verify that an image is a true vector at high resolutions being that screenshot cutouts and other non-vector images will show pixelization and other issues if you try to export them at high resolutions.