Depends on your setup. I am going with a 15 gallon Keg for my HLT and BK. Because I have them. I plan to use my 15 gallon HLT to heat my recirculating mash and the extra thermal mass will be welcome to help stabilize the temp over time.
But, if you think about it, you won't ever need more water than your batch, because you are (likely ) going to split the water into mash and sparge volumes. 5 gallons is fine for this.
I think the biggest question I have is are you actually going to use a cooler for your HLT? If so, I'd recommend a cheap kettle of some sort instead.