Width is more important than height, as budgies (and other birds) like to be as high as possible. Flight cages normally have a good amount of both height and width, and are a nice rectangular/square shape, which is best for a bird cage. For a single budgie, I would look for something like this, or if you have a tighter budget then this is also a good sized cage, with much more width than height. Also, make sure that there isn't more than half an inch for bar space.

I’m currently using this for my budgie. He’s happy about it so far and it’s also affordable. Yaheetech 30-inch Rectangle... https://www.amazon.com/dp/B08LVPHZXY?ref=ppx_pop_mob_ap_share