Attachment meta size should return file size instead of duplicate dimensions

[trwnh]

Pitch

Make Attachment#meta[original,small][size] return file size instead of {width}x{height}.

Motivation

Dimension data is already available with meta[original,small]width and meta[original,small][height]. Also there is no way to know how much data an attachment will consume before downloading, which is not good for users on capped data plans.