The most commonly used bulb for the model fairground is the LES screw thread based lamp. The glass of these is 5mm diameter – so a little oversize for 1/24th scale but fine in the larger scales. Wire ended bulbs of similar size are also available.
For fairground models I have standardised on 14volt 0.05amp bulbs. These produce more than enough light for most models and the 14volt rating if used on a 12volt AC supply gives quadruple the life of a 12volt bulb on a DC supply.
In designing the lighting on a model you should aim to need less than 20Amps in total – at 12volts this equates to 240Watts which will generate a fair bit of heat. 20Amps worth of 0.05Amp bulbs = 400 bulbs.