Size of blade depends on how thick the metal you want to cut is. Fewer teeth pr inch, the quicker the blade cuts, but less control. The more teeth pr inch, the smoother the cut and better controll.
For minimum controll you need at least 3 teeth = the thickness of the sheet metal. If you are cutting 0.1" thick sheet metal you need at least 30 teeth pr inch.
The blades I use most are:
nr.3/0,very corse,good for metal thicker than 1/6"
nr.0, medium, good for 1/16" to 1/32"
nr.0/3, fine and nr.0/5,very fine for metal less than 1/32"
Best regards
Rolfkt