Bill:

The simplest thing would be just to get a 3-view of the C-336 or 337 and size yours in proportion.

The tail moment and area proportions are the same for a multi as for a single - the air doesn't care how many fans are blowing. With conventionaly mounted multi engines it is not a bad idea to make the vertical fin and rudder a bit larger than for a single, with centerline thrust the normal size is fine since there's no chance of off center thrust.

Bill.