When we talk about length we are referring to the length along the cutting edge of the blade, this is traditionally given in inches.
The Gullet is the area between the teeth. When sawing soft and coarse wood a lot of saw dust is produced so the saw teeth need bigger gullets to keep the saw moving freely.
When cutting metal the chips are much smaller, so the saw can have more teeth per inch to cut with. If you tried to saw soft wood with a metal saw, the gullets would jam with sawdust, making the job very difficult indeed.