You won't need heat past three weeks, if that long.
I use red 75, 125, or 250 watt bulbs, depending on the season. all I could find locally was a 250 watt bulb, I had to go online to order them. All incandescent type bulbs radiate heat, that's why they are used to provide heat. Many of us use red. Chicks or chickens can become cannibals, especially if they see blood or raw sores, the red color is supposed to hide that color. Other colors, including white works too. What you are after is something that provides heat. Some people use dimmers to reduce heat input.
At 4'-9" you can heat one end and let the other cool down to ambient as long as the heat source isn't too strong. You would need ventilation up high so the heat can escape. 1'-9" is plenty tall enough. The size should be big enough to keep 6 bantams longer than you need to temperature-wise. Whether your coop is ready is a different question.
By the time they are 2 weeks old they will easily be able to fly more than 2' up. Well, not the silkies because they can't fly. You might want a frame with hardware cloth to fit over that. Some of those holes in the sides look big enough they can get through them too so the hardware cloth there is a good idea. You are talking bantam size chicks.
Instead of a heat lamp you can use a heat plate or heating pad cave. Those would work well for 6 bantam chicks. Some heat plates are only rated for above 50 ambient but you are well covered there. A lot of people really like the heating pad caves. Those are made with the kind you use on your back but you need one that does not have an automatic shut-off. Blooie has a nice thread on here on how to set up the heating pad but I lost my links and photos in a recent computer crash so I can't help you with that.
