Enlarging the negative. How big?

Depends on how crisp the negative is! If it is razor sharp there is no reason why you cannot enlarge it to its maximum. A thirty-five millimeter negative should be able to produce a 24 x 30 centimeter print with ease and in exceptional cases 50 x 60 cm. To check if the negative is of a good enough quality, enlarge part of the frame to 18 x 24 centimeters, if it looks a bit grainy the chances are that when the whole negative is enlarged to 50 x 60 cm and viewed from a meter away it will look good.

When enlarging to larger sizes you need to take into account the distance that people will see the picture from. Large photos are rarely viewed up close.