If sprite have 1 animation frame, preview works fine, but if there's more than 1 frame sometimes animation preview is cutting out right side of the image. I have 5 sprites with few frames and only in three of them animation preview is not showing full images - rest two are fine though.
Simple option to be able to scale that window/panel (that saves changed size after closing it) should fix that issue.
and play/repeat button would be nice addition ;)
off topic question. Is it possible to have that preview window open all the time, like other "animations" and "animation frame" windows? and instead of clicking RMB on animation name and selecting preview maybe something like this :)