I'm not a luthier but I frequent a lot of DIY guitar forums........ Not entirely sure about what I'll be saying so the professionals like empty71 might wanna correct me if I'm wrong
You would want a body blank with dimensions at least 19" x 14" x 1-3/4" (Length x Width x Thickness). With those it's quite a tight fit already for a normal strat-shaped guitar. If you're looking to construct a guitar with more radical shapes (Explorer etc), you'll need a larger body blank.
For a neck, it depends. Are you building a bolt-on, set-neck or neck-through? Also what kinda headstock are you planning on? Generally if you want a normal strat (read non-angled) headstock, you'll need a neck with dimensions of at least 27" x 4" x 3/4". If you're constructing an angled headstock, you'll need to make a scarf joint near the end of the neck towards the headstock. That may require more wood. As expected, set-neck will requrire an even longer length of wood, and you most probably will have to glue another block to make up for the thickness. Neck-through will require an extra length of wood at least the length of your guitar body, advisable to have a piece even longer than that in case anything happens.
Fingerboard-wise, it depends on your neck scale. Obviously the longer the scale, the longer fingerboard you will need.....
Lastly, about the grade of the wood. It more or less depends on the company you're buying from. Their grades of wood may differ slightly from other companies, but only by a bit. I doubt that their customers won't be able to see the difference between AAAAA flamed maple and AA flamed maple
Also, one thing about wood. Since you're making a guitar, you'll want wood that is either air or kiln-dried with a moisture content of below 8%. If you're buying from a website that deals mainly with tonewoods for musical instruments, then it's all well and fine, but if you're buying from a normal lumberyard, their wood may not be suitable for building a guitar.
Got a small question though, do you have all the necessary equipment to construct a whole guitar from scratch? There are a lot of tools needed all the way from the planing process to the routing and finishing..... Well maybe not finishing but the rest require equipment that may not be easily accessible locally, unless you're studying and your school has a well-stocked D&T facility or something.
