Belts don't need to be really tight. Some users leave some slack so they can use the right hand to slow the machine down somewhat, especially if they are using a clutch motor. A little slack is also good if you happen to pull the needle to one side, breaking it, and causing a jam. Then you don't have to rely as much on the safety clutch to protect your machine. Generally there is enough adjustment on the motor bracket to allow you to tighten the belt as far as needed. If the belt is not the right length, you won't be able to adjust it. Vee belts should not ride on the bottom of the pulley. The top of the belt should generally be flush or nearly flush with the top of the pulley. It is the friction on the sides of the belt that transmit the driving force to/from the pulleys. At the mid point between pulleys, you should be able to depress the belt about an inch with firm thumb pressure. Should not be hard to do. It is not like vee belts driving fans, ac compressor and alternator on a car. They need to be much tighter due to the amount of power they have to transmit.
Tom