On body metal, I use a 3" diameter x 1/16" thick abrasive wheel on an air-powered rotary die grinder---this is a bit tricky because I grind with the edge of the abrasive wheel at 90 degrees to the weld, and it takes both hands to contol the grinder so you don't gouge the parent metal. When the weld is close to flush, I use a 36 grit disc with semi-flexible backing pad to finish the weld down flush.
On something like a frame weld, I use a heavy abrasive disc 7" diameter, on a heavy duty right angle grinder and knock most of the weld down, then switch to a 36 grit sanding disc on a semi-flexible backing pad to bring the weld down flush.------if you are grinding frame welds, be sure that the metal you are grinding has been V'd out to almost its full thickness before you start the welding, or grinding it flush will remove all the strength from the weld.
You can do it with a small grinder, but its very time consuming.
One of the best sanding disc thingies I have ever seen has a circular shape, but is made up of many peices of what appears to be about 36 grit abrasive coated heavy fabric, and all the peices overlap each other, kind of like a fan of cards.----I don't know exactly what they are called, but they cut very fast, and blend excellently.