Sewing machine motors are typically very low wattage (mine was 90) and run fine on a transformer.
For anything else, look @ the info that will be somewhere on the machine. Add at least 10% for potential surge (so if it says 500 watts, you need a transformer that handles at least 550). If the maximum wattage on any machine was 500, personally for the miniscule price difference, I would get a good 1000 watt transformer.
Your mileage may vary, but I always have had good luck using a transformer with my US kitchen appliances.