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Usually, even for larger motorcycles, they try to keep a 2ft length, just to prevent the flame from going back to the valves in case of lean burn.
The diameter really depends on the CC.
For a 300cc single cylinder, the tube dia should be about 1 inch; so I presume that 3/4 would be good for a smaller one.
You can get cheap mufflers online, but should you have the time to make one, just drill a hole in a 6-10" dia tin can (off center) on both the entry and exhaust side, stuffed with rockwool, and chicken wire to keep it at it's place (or a perforated pipe), and weld the can on the exhaust pipe.
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