You can keep your ackie in a 20ga, the rest of its life. If you provide all the right choices. But, a 20 gallon is very limiting with what you could allow. And its not very enjoyable for you.
The size of a cage is not a standard thing. If you understand your monitors needs and as importantly wants. With a larger cage you "can" add more to improve the life of your monitor.
If you improve the life of your monitor, it results in more learning and entertainment for you. The more you see, the longer you will maintain interest in keeping your monitor.
Some species of monitors spend lots of time and exercise in trees, some species spend that time in the ground, burrowing. Some species use water. Some species use all of those. Most species include some of these areas.
Ackies are burrowers first and climbers second. They don't voluntarily use water.
Why I mention this is, you said you have a cage for when your ackie grows up. Yet, you did not say if that cage provided the ability to allow lots of burrowing. For ackies, thats what they do. So no matter what size that cage is, it may not be suitable for the number one perference of ackies, to burrow.
A long cage is great because it can allow a cluttered area, lots of places for the monitor to hide, with an open area. Monitors like to sit out near their hiding spot and watch over a large open area. This gives them a view and the feeling of safty. Also a long cage can provide a very good heat range.
Height in a cage is good for providing climbing areas. Like above the cluttered area and above the open area.
Depth is not as important as the first two, it does provide more square footage(square inches)
None of those dimensions provide depth below the level of the door. An area that holds substrate to allow burrowing and nesting. For ackies it should be a minimum of a foot, but two feet is better.
So when you or others ask about how big a cage, I often do not respond, because it doesn't matter how big the cage is, if you do not understand how to make that space useful to the monitor. It doesn't matter how big the cage is.
To add a very important dimension, People think temperature choices are provided lenghtwise(as mentioned above). That is, from one end of the cage to the other. As in, a hot side and a cool side. The problem with this is, monitors do not use that method first. They perfer to regulate temps by moving up and down. Up to bask in the sun, or on the warm(hot)surfaces heated by the sun. Down to get to cooler places in the ground or down in trees hollows, or even down in water. They use vertical temperature selection(hence retes boards) They can move horizonally for temperature selection, but is normally a second choice.
For you the keeper, you should provide both choices.
So the answer to your question or anyones question about size of a cage, it needs to be big enough to provide needs and wants of the animal enclosed within it. Cheers