The initial feature of an budgie cage you will need to consider is most likely the size. Budgies are really active and then get almost all of their exercise from flying. If your budgie will spend most of its time in a cage then you should be saving up for a flight cage. You can go for a smaller size if it will be out of the cage for a few hours each day. As they need to be able to fly to be physically and mentally healthy you really want a cage that is longer horizontally rather than a pretty upright, narrow one. Some very pet supplies online
like bird cages are nice to consider but won't offer you a decent home for a budgie.
Listed below are some minimum sizes, listing the height x width x length. Any time you mulitply these together you obtain the volume belonging to the cage.
The minimum size for a single budgie would be: 18x18x18 inches, which gives a volume of 5,832" cubic inches
(46x46x46 cm = 97336 cubic cm)
Minimum size for 2 budgies: 30x18x18inches, which gives a volume of 9,720" cubic inches
(77x46x46 cm = 162932 cubic cm)
Minimum size for 3 budgies: 32x18x20 inches, which gives a volume of 11,520" cubic inches
(82x46x51 cm = 201756 cubic cm)
From there you can work out the minimum size by having a volume of 3800 cubic inches (62271 cubic cm) per budgie.