What is the belt size for this sander?

It takes a 5-inch-wide belt. Check the belt length marked on the machine or in the manual before ordering replacements, since 5-inch belts come in several lengths.

Is 280 rpm fast enough for sanding?

It is on the slow side. That makes it controllable for flattening and softwood work, but it removes material more slowly than a typical 3x21 belt sander.

Does it have variable speed?

No. It runs at a single fixed belt speed, so you control the cut with pressure and belt grit instead of a dial.

Can it be used for metal or only wood?

It is built for wood. Metal sanding generates sparks and heat, and this sander has no spark guards or metal-rated dust handling.

Does it collect dust?

There is no clearly specified dust collection port or bag. Expect to wear a mask and clean up chips yourself.

A hobbyist who needs occasional rough sanding and wants a simple corded machine. Someone sanding daily should look at a variable-speed 3x21 sander from a major brand.