Is the bonnet on a 3 series e36 BMW the same size on the coupe and the 4 door models?

No the coupe and convertible share the same bonnet and front wings but the 4 door cars have a different bonnet/wings.....the grill holder (complete panel that goes under both headlights aswell) is the same on all models albeit with revised kidney shaped grill vents on later cars. Hope this answers your question