Hi:
I would say the handle length should be whatever is comfortable for you for that particular tool.  A big bowl gouge would need a longer handle than a small spindle gouge simply because you need more leverage to take big bites of wood, but the small gouge should still have a handle long enough so you aren't making major changes to your posture when switching between tools.  If you are comfortable and accustomed to holding a tool one way you don't want to be forced to change simply because the tool handle is shorter on the next tool.

As far as material goes any good solid hardwood should be fine.  My Sorby roughing gouge has an ash handle and my Henry Taylor gouges have close grained handles, maybe maple, birch or beech.  That said, I have a little 1/4" spindle gouge that I needed a handle for and didn't have any suitably sized hardwood around so I made the handle from a length of spruce 2x4.  It seems to work fine.    My favourite handle that I made myself is made from a piece of dogwood.  It is a wonderful wood to turn.

Good Luck!
Bob

I have used ash, oak and walnut for tool handles.  Hickory is prized for its shock resistance so it would be an excellant choice as would hard maple.  I would probably not use softwoods or stuff like soft maple, box elder or poplar although they should work fine on smaller tools.

Length is matter of personal preferance as wall as intended use.  If you are making a handle for a tool that will be used well beyond the tool rest (1 1/2" scraper or bowl gouge), you will want a longer handle for the leverage advantage (as well as added weight).  The handle for a skew, especially if you are rolling beads would be easier to handle (no pun intended, really) if it was shorter.  Unless, of course, you like the handle longer.

Here's another rule of thumb: the handle should be 5 times the length that the chisel will extend past the tool rest. So, if your going to hang the end of your chisel 3 inches over the rest, your handle should be 15 inches. Can't remember where I read that (probably several places), but it has been a useful guideline for me.

I've been using recovered hardwood from shipping pallets to make handles for my tools. No two are alike, which is helpful, and I've ended up with some interesting tropical hardwoods that you'd probably not want to otherwise pay money for only to turn them into tool handles.

Don