Even simple things could take us some time … And that was the case for me, to figure out best size of pictures which I could use to my bootstrap carousel. The result size is 750 px (width) x 350 px (height) and it really works for me! If you would like to use wider photos, the height shouldn’t be much bigger, but you always could manipulate width – make it wider (panoramic photos). Always have in mind bootstrap grid (12 columns) and the amount of pixels it’s taking per one column.
It’s good to know, that Google image search is allowing us on searching exact size of pictures, so you could use them to set up your prototype having exact size you are looking for with a good quality.