Thank you @Marc_Carlyon & @OboShape for trying to address this.
Definitely no warning with my initial ~4Mb GIF attempt either and this warning would be useful.
If this warning can inform the user of Max file size too it would be great.
As you can imagine, it takes a little effort to tweak a GIF to (file) size and if the user knows of a size to aim for in advance - this would be a good start.