50W carbon dioxide laser tubes are reasonably compact, only about 3 feet long. The thing they cut best is acrylic because that vapourises. You can cut paper, cloth and thin wood. If your material gives off lots of stinky smoke, the glue in plywood is particularly bad, you need to force air down the beam to disperse it.

80W tubes are a lot bigger, if 100W tubes are not getting silly size wise then 120W tubes most certainly are. Check them out on e-bay, see what they reckon the various tubes will cut/mark. Lasers are fun with acrylic but can be disappointing with other materials if you want instant gratification.

With 120W you might just be getting to the point where it would cut wafer thin metal, a solder paste mark perhaps, but a minimum of 1000W, probably with oxygen assist, is what you see in laser metal cutting movies.

At low power, everything metallic the beam hits has to scatter the light, if it reflects then it will not so much as mark the surface and it would be extremely dangerous. Even the 50W tube unfocused will blind you faster than you can blink. There are coatings you can apply to help mark metal surfaces but it is marking, not engraving, there is no real cut unless you have the power.

Have fun, stay safe.