how much water to sparge with?

I have an 8 gallon boil kettle and am stepping up to a 15 gallon kettle and 15 gallon mash tun. Is it necessary to step up to a 15 gallon HLT as well?

It depends on how much you use for mash water, grain (for absorption) and what you want for your preboil volume.  To fit maximum sparge water you may have to increase your mash water volume some. 

Mark

The purpose of an HLT is to free up your boil kettle so you can run directly from the MLT into the kettle. 

You need an HLT large enough to hold your entire sparge volume.  It is possible to get away with less..but, it's a big pain...and mostly defeats the purpose.

The minimum practical size of an HLT is your finished batch volume.  So, if you make 5 gallon batches at a minimum you need a 5 gallon HLT.  For high gravity beers 2x batch size is a better number.