I am a new bearded dragon owner

Question
I am new bearded dragon owner, and the guy that I got her from is amazing with the animals. He told me I need to get a 20gal for my beardie. She is ( i think) 6 months OR SO, and she is 7inches. I thought the breader said because I have been temp using a 10gal for her. because I am waitting to a new cage.

I know the diet care and handeling care. and NOT to use sand for a wile. He told me to use pp-towls for now because he can eat the sand and die. Should My Lady Beardie be bigger than 7 Inches by now? it is bad that I have not gotten a 20gal yet?

how deep should her water dishe be? she likes to sit in her water too, is that ok?

What size cage should I have for her to be a good owner?

I hold her a lot, and so do my kids. She loves the kids and the dog and they love her too. She is an amazing pet and we just want the BEST FOR HER.

PLEASE,
HELP GIVE US THE ADVICE TO GIVE HER PROPER CARE. THANKS

Answer
20 gallon is minimal but ok, a 40 gallon breeder would be better.  You need floor space not air space.  Are you providing the proper lighting?  110 degree basking spot AND VERY IMPORTANT!!!! UVB!!!!  You must do the UVB or the dragon will get sick.  The light bulbs are expensive and must replaced as per directions on the box but they ARE necessary.  You can use indoor/outdoor carpet if you want a better look in the cage.  

I hear the "no sand" argument a lot, and perhaps there is something to it, but I have to wonder how the species survives in the desert if sand is a bad thing...  I think the nonnatural clumping sands are the problem.  Natural sand shouldn't be...  Oh well, better safe than sorry?  

Sitting in the water is fine.