This is basically a junk crossbow that I've been making.  It's in an African lockbow style.  I've got a couple of pictures here, but the flash on the camera is intense and the pictures aren't great.
  Anyhoo...

First picture is from the side.

Second picture shows the bow and a single quarrel.  The entire bow is ash - the prod, stock, trigger, and wedges are ash.  The string is three ply braided cotton - I just picked up any bit of string that would take the strain.  The quarrel is hazel, "fletched" with an ivy leaf and willow bark cord.  The bow hasn't been properly tillered, the stock has not been smoothed, polished - just left alone.  And here's the thing - this lockbow blasts the little quarrel a clear fifty metres before starting to slow down.  Drawweight is 94 pounds.

I couldn't give you accurate dimensions.  I believe that the prod is around 16-17" long, drawing a between 7-8".  I've been measuring this on hand spans and working it out from there, so... it's quite narrow, but thick.  I expect it to take some fairly heavy set, but I don't mind.  It was a spur of the moment kind of weapon.

Well, I can't give you hints, really.  It's not much to make a large crossbow prod, because it's basically all about having as much weight as possible, whilst ensuring that the middle doesn't bend as much, or more than, the tips.

Nobody did.  You don't need teaching.  All you need is to read a little bit around the subject, gather as much information as you can.  The only thing you really need help with is the mechanism, and that's simple if you know how.  This entire lockbow, humble though it is, was made entirely with a pen knife.

Yeah.  Kids can be pretty ingenious when it comes to projectiles, can't they?  I know I was.

That's a shame.  There are several ways to stop a bow breaking, but I can't cover them here.  I'd go to a library, find some bowmaking books, and cram.  Or check out various posts here, and at PaleoPlanet.  It's easy when you know how!