Can the small dubia roaches get big or do they stay that size? I'm trying to figure out what Dubia roaches I can feed a baby bearded dragon?

yes

My baby beardie mostly eats small (1/4 inch), and occasionally he'll eat some that are 1/2 an inch.

But as long as the food item is the length of the space between your beardie's eyes, you're safe.

And, small dubias do grow, as they're just baby dubias. But from my experience, depending on how much you get, you'll feed them off before they grow too big.

They way you measure if the food is the right size is...

If the bug is the same length as, or is small than, the distance between the dragon's eyes then they can eat it.

My guess would be that a roach is a bit big (Though we don't get them in the UK so don't take my word for it)

I would probably recommend small crickets/locusts at first, then larger crickets into adult locusts/ roaches as your dragon grows :)