If you can an 18x18x24 is the best. I have a 12x12x18 and my whites gets a little restless at times and seems like she dosn't have enough room. But if its all you could aford at the time, get the smaller for now. Its much bigger then what you have and will help the little one settle down to have the extra space.

You may try the terrarium forum here for some pointers about tanks. There are several people who have converted inexpensive horizontal tanks into large vertical tanks for a lot less money then it would cost to buy the larger tank.