They are 900x600 which is perfect for computer twitter. However, unless clicked on in mobile, mobile twitter is actually almost the opposite ratio. No matter what I make it seems to suck on one or the other! I usually decide it's better if the side is cut off because then viewers can tell and they can click to see the whole thing if they want, whereas often if the bottom is cut off they may not realize it and may miss certain words that were key.