We reproduced the problem. And indeed Files app doesn't show file size for .artstudio files stored locally.
But it shows correct size for some other documents, for example for Pages documents.
Unfortunately it seems it is managed by iOS system, so we cannot fix that.
More details: Artstudio Pro uses "Package" mode for .artstudio files, which mean document is stored as a folder, not a file. Files app generally doesn't show the size of folders.
'Package' is used by apps like Pages, Numbers, Swift Playgrounds and few others. iOS system recognizes some 'package' files and show correct file size for them (Pages, Number). But it doesn't recognize all the rest: .artstudio, .playground... We cannot fix it, we defined .artstudio extension correctly, we cannot do anything else to fix it. We can treat it as a bug in iOS 11. iOS 12 has the same problem.