When I was trying the mobile UX, at the file upload, after waited a long time to complete the upload, I’ve got the Alert saying that my file size was large.
It would be nice if we are able to control the files size and alert before starting the upload. It would save time and resources and give better UX. Specially for mobile users but not limited to!
Not possible through browser to my knowledge. Literally impossible.
I understand 
May be a small text next to upload link could help like “Max 3.07mb” for mobile,
and “Max. File Size 3.07 MB” for Desktop file upload modal.
We unfortunately can’t do that because we automatically try to downsize images until they meet the size requirements or we spent too much time on the server trying to… Since there is no way to know the size of an image after downsizing before actually doing it, the client has to wait for the upload to finish and the server to spend time trying to downsize the image.
The only way we can fix that would be to do the downsizing operation on the mobile before uploading the image. This is on my list but near the bottom at the moment.
This would be quite fab. Enough that I’d be willing to contribute 
to help make it happen. Relates to an earlier post of mine from this summer about problems facing many of my community members.