You could take the chef approach by picking straight banana's and squaring them off first. The sizing would be pretty consistent, the leftovers can go in a shake, or frozen for a smoothie later and you would have a higher percentage of perfect coverage :)
+1 for freezing leftover bananas. Freezing ripe bananas just before they over-ripen is the greatest life hack I ever learned. I can't believe it took me so long to stumble on it.
Slice ripe bananas into a reusable sandwich bag, pretty much in a layer (not a frozen clump) so you can break off a part of the layer, and freeze. Later semi-defrost and throw in a smoothie or protein shake. Never throw away an over-ripe banana again.
