Need help with dimensions

[Curtis72]

So im trying to place a order with HQ aquariums (formally known as Pic0 Aquariums) but I am having trouble deciding on what my tank dimensions should be. I normally wanted something like 19x19x9 but my current tank is 18x18x18 so basically im just making my current tank shorter. So im asking you guys what dimensions would you get because at this point I have no idea. :headwallblue:

Thanks ahead of time!