I think the OP is talking about a brooder for chicks, the first reply is in regards to an incubator.
For a plastic bin, you can use a 125 if it's pretty far from the floor of the brooder and the top is off. A 100 will work too, just a little closer to the brooder. We start with 125, then switch to 100's after 2 or 3 days, and down to a 60 after a week or so depending on how the room temp is where the brooder is located. I have heard of using a dimmer, but I've also heard of problems with dimmers and higher wattage bulbs. I've never tried it - might work great!
Just remember, the entire brooder doesn't need to be hot. There needs to be a slightly cooler area where they can get away from the heat, so concentrate your bulb on 2/3 of the area. I like to put the food and water in between the warmer and cooler area at first and then move it to the cooler area after a few days.
Watch the chicks and you'll figure out what they need pretty quickly. They'll get right under the heat at first, but after a few hours they should be moving around diff. areas of the brooder if they are warm enough.