Facebook uses a grid tool to determine if a post has 20% text or less in order to be accepted as a boosted ad. Note that the same amount of text positioned slighter higher or lower could cause it to cover more than one area of the grid, thus exceeding the 20%. See example below:
The recommended image size is 1200 x 628 pixels. Keep in mind that on the event page the date and image title will overwrite the image so it is best to leave that space blank or as an image that will support superimposed white text.