How a log becomes a blank
For a flat rim or natural edge bowl, the log is split in half lengthwise through the pith, and each half lies flat side up. The bowl's circle has to fit across the log and along it, so the biggest bowl is the smaller of the log's diameter and its length, and it can be only as tall as half the log's diameter. Each half holds as many circles as fit along it: a 12" × 24" log gives two 12" blanks from each half, four in all.
For end grain work (vases and hollow forms), the log stays whole with the grain running up and down. The form can be as wide as the log and as tall as the log less an inch off each end, where logs check. The pith stays in the base, where cracks start, so keep the walls even and dry it slowly.
Length runs along the grain. It's not the bowl's height: that comes from the log's diameter.
Walls, bases and green wood
The thickness guide is a minimum: 3/8" walls for bowls up to 8", 7/16" up to 14" and 1/2" beyond, with a base 1/8" thicker than the walls. Interior diameter and depth are what's left inside.
Wet wood shrinks and warps as it dries, so green bowls are turned twice. Rough turn with walls 10% of the diameter plus an inch thicker in all (half of that on each side), let it dry for several months, then turn it to its final shape. Expect it to go a little oval as it dries.
A worked example
A 12" log cut 24" long, split for flat rim bowls, yields 4 blanks, each up to 12" across and 6" tall. With 7/16" walls and a 9/16" base, the inside can be 11 1/8" across and 5 7/16" deep. Turned green, rough it with 1 17/32" walls.
Before you mount it, check the Lathe Speed Calculator: a 12" blank starts at 300 RPM.