The most important thing about making a gif is making sure its fairly small so it loads fast enough when its inserted as a custom thumbnail (in a grid of thumbs). If its too big that thumbnail/media item will take way too long to load with a grid of thumbnails on the page.
In most cases the thumbnail file of the gif won't be any larger (dimension wise) than 600px wide. This allows you to make the gif smaller but maybe longer in duration.
These sites and tools below are useful in making a gif if you aren't sure how.