Most mortisers are equipped to handle 1/4", 3/8", 1/2" and 5/8 square chisels.
Converted to metric: 6,35mm, 9,5mm, 12,7mm and 15,9mm.

Some are even capable of handling a 3/4" chisel, but keep in mind that this is a pretty healthy chunk to cut at one time and may require a lot of physical exertion. Remember that even if you're cutting a mortise that is 3/4" wide, you don't need to cut the entire width in one pass. A few cuts down one side of the mortise and then back up the other with a 1/2" chisel will be just as effective and not nearly as taxing on you as the operator.

Advantages:

• Faster than a chisel by hand
• Consistent in size
• Strong joints
• Reliable joints
• Looks good
• Large glue surfaces enhancing the strength
• Convenient