I have done a few kit brews with good success. Next I want to try to go all grain. I have been doing some reading in books and online but am still confused about how much grain to use for a batch and how much sparge water to use. Is there some where that I could find formulas for these? Do I have to limit myself to 5 gallon batches? What if I wanted 10 or 15 gal?
Before you build or acquire the additional equipment needed for AG, you can familiarize yourself with mashing by doing partial-grain brews. Start by steeping specialty malts like crystal, then try doing a "mini-mash" with just a couple of pounds of base malt to get the hang of things. You can easily do that sort of stuff using a standard 2-3 gallon stock pot or crock pot. Alternately, find a more experienced all-grain homebrewer and have them walk you through the process using their system.
Any good homebrew book will have formulas for calculating mash and sparge liquor volumes. As a rough rule of thumb, you use 1-2 quarts of water per pound of grain, depending the desired "thickness" of your mash. Different styles, mashing techniques and ingredients might demand a thinner or thicker mash. Sparge water volume is roughly equal to total batch volume minus mash water, plus allowances for loss of liquid during the boil, water retained by grains and whatnot.
Maximum batch size depends on the size of your system and desired OG for the beer you're brewing. The bigger your system, the bigger the batch you can brew, but if you're brewing a big beer, you collect less wort so that it's more concentrated. Since AG brewing requires more time, many brewers step up to 10-15 gallon batches when they go AG, since it doesn't take that much extra time to grind, mash and sparge larger quantities of grain.
It's relatively simple and cheap to step up to AG brewing if you just do 5 gallon batches, or perhaps a 10 gallon batch split two ways so you can still use a 5-7 gallon stock pot and perhaps a stove-top burner. Going to 10+ gallon batches requires more time, money and planning to get a system that works well.
Scaling up means that you have to rethink how you brew, due to the safety factors and thermodynamics of heating, cooling and transferring larger volumes of wort. You might also need to rethink how you handle your yeast and how your ferment and store your beer due to the increased volumes. Going big also means less flexibility in what you brew and bigger potential losses if a batch goes bad. Are you likely to drink 15 gallons of a particular beer?