Update June 2017: Tumblr dimensions/sizes for optimal display of your GIF on the dashboard. To prevent your GIF looking fuzzy/ not animating/ refusing to load, use the sizes above.
Most important is the width, height can be whatever you want up to a point. GIF that are next to each other must be the same height or tumblr will crop them. 3MB+ will not work. More on tumblr sizes