You could divide it until he/she grows into it.
But it might like the space to explore!
I get mine to hunt for his food too...sprinkle a couple on one side of the tank then a couple on the other side. Sometimes he stalks them other times he just runs full speed right at them!
If I just drop them in, he'll stand under the cup and catch them before they hit ground!
Aquariums are usually narrow...email or ask for the dimensions.
When your beardie is larger it needs to be able to turn around without rubbing it's tail on the sides
A 40 gallon breeder tank is what most people use, it's around 16" wide.
Personally, if i was to do it over again, I would build a custom viv.
But in my circumstances, I was getting a rescue, I needed something fast, so I went to Petsmart, they had a large, Zilla 40 gallon bearded dragon tank kit on sale for $159. I got the right 24" Repti Sun 10.0 bulb and fixture on line.
If you wait for a "$1 per gallon" sale you could get the same tank, just the 40 gallon Breeder Tank for $40!
Another thing to consider, you might want to consider feeding him/her in a separate 10 gallon tank or something other than their living quarters, either a small tank or when it's bigger, a tote.
I have been feeding mine in his tank, I cleaned it out today and found 4 crickets hiding under the reptile carpet. The last time I fed him crickets was 4 days ago when I ran out of Dubai roaches.
Today, I cut a large piece of tile that fit half his tank and when I fed him, I saw some small roaches crawl under the tile! I have it so it's removable!
I am either going to try and cut up some reptile carpet or felt to put in the tiny cracks between the tile and the glass, or start feeding him in a small tank I have.