Best size gouge for removing waste

[abrakadabra]

Walnut is really hard to cut wood.
I wouldn't use anything bigger than 10mm (3/8").

I would recommend you to buy a small drill press and make a lot of small holes. Then you can remove a lot of waste using a carpenter's chisel with a mallet.
A carving gouge is a fragile tool for detailing.

Harbor Freight sells a small 8" drill press for $40 when it's on sale. But don't buy their chisels.
Lee Valley has nice and cheap Czech chisels Narex Bevel-Edge Chisels - Lee Valley Tools

[Donsexton]

I would use a 1" ball monster carbid bit on a die grinder. Then finish it with a 1" flap sanding wheel. If I was going to go with a gouge it would be a 1" short bent spoon in a #7 sweep to gouge the depth and a 1/2" back bent #3 sweep to finish the inside walls.

[Nomad]

Well, I generally prefer a spoon knife (often called a crooked knife) over a gouge for carving bowls on spoons. The ones from Pinewood Forge are about as good as they come but if the bowl is deep, I like to use a North Bay Forge spoon knife. The large handle will give you a lot of power for carving.

These are North Bay Forge crooked knives.

[Gudirutta]

Thanks so much for the tips! I'll put up a pic when I'm done and note the tools used.

[Brittany]

A hook knife by Pine wood forge. Great for scoping out spoons. Remenber that you use it side ways.

Charlie

[RUssL.]

A good size (or two) Forstner bit in a drill press removes waste like you wouldn't believe if I can't get a router in the spots to do it.

My, how my aesthetics have changed over the last couple of years! :-)

RussL.