Mine have been in a 4ft since they were about 4-5 months old and have been absolutely fine. I did have my first one in a smaller tank for a few weeks until I got the bigger one set up, but they grow so quickly that they will soon outgrow a smaller tank. If you're concerned about it being to big for a baby then you could pop a divider in and then as the Beardie grows you can take it out again. Much cheaper way of doing it.
I must admit I do love my beardies and you can handle them alot more than Leos and they sleep in the most amazing positions at times too lol