A 1HP 12 inch disc works well.
I started out with the little bench top 8 inch disc 4x36 belt sander. While it did okay, it was slow and stalled with little pressure.
What size metal are you working with? Anything under 3/16 would be okay with the little 4x36 belt. Keep an eye on the local classifieds they come up pretty often for $20-$40. Oh yeah, be warned that these are not made for metal work, the plastic pieces will melt rather nicely when you are throwing sparks all over the place.
Once you are tired of fighting with the small machines, step up to a 2x72 belt grinder, they are a treat
While not exactly a bench top sander, they are well worth the shop floor space.