Something to keep in mind when shopping for cages - even though your bird isn't going to utilize the bottom half of the cage as much as the top half, that doesn't mean the lower space is completely wasted.
Birds appreciate height. It provides them with a sense of safety to be "high". It is natural for them to seek out the higher perches and hang out in the upper reaches of their cage because it feels more secure. If you give them a cage that doesn't have any height to it, I suspect that they will not be as happy as they would be in a cage that allows them to get up higher. It might be necessary to make a very long and low cage for a disabled bird, but the average flighted bird would rather be up. You can also arrange the cage in such a way that all areas get utilized for different activities, like bathing, eating, foraging, playing, etc.
However, since they are going to be spending the majority of their time in the upper reaches of the cage, you can't consider all shapes as completely equal when comparing usable space. A cage shaped like a skyscraper gives the bird some nice height, but very little flight-space and it makes it hard to find a good spot for perches and toys. If you had a cage that was shaped more like a cereal box, it would have height AND length. This would allow you to set-up the interior in a way that lets the bird fly across the middle and also arrange perches so the cage stays cleaner. And the bird can enjoy being up high, when he needs to feel more secure.
If you are looking for a large cage that is longer than it is tall, you will probably need to customize your own cage. For example, by modifying two flight-cages so they are connected on one side. You can also find some cages that have removable dividers.