I assume this shed is just going to be used as a brooder and you have another coop? As 3rivers said there are way too many variables for a short sweet answer. Your climate, the weather you can expect when brooding, and how long you plan to leave them in the brooder are important variables. How hard you are willing to work plays into it also. The more chicks you put in there the more they will poop, so you may be working a lot on poop management.
What are your goals? Are you trying to maximize the number of chicks you can raise in there or do you have a top end number in mind? Are you going to be regularly turning them over, starting new chicks as soon as one brood moves out or will there be a rest in between? Just trying to get a handle on why you are asking.
There are a lot of different ways to provide heat, you are not restricted to heat lamps. Each method has their advantages and disadvantages. I use heat lamps and brood in my brooder in my coop even with outside temperatures below freezing, but I never brood more than 30 chicks at a time. My brooder is 3' x 6' and pretty well enclosed in winter, pretty wide open for ventilation in the summer. In something as open as I envision your shed to be that may be more challenging. A heat plate or heating pad is limited in size so limits the number of chicks they can handle. One method I'd suggest you investigate is a hover.
Envision a shallow open box with the opening facing down. Raise it enough so the chicks can get under it and provide a way to heat it from below. A standard heating method is a heat lamp. The box traps heated air underneath so the chicks go under there to warm up when they need to but have the rest of the shed and the top of the hover to play in and for food and water. Many people are surprised at how much time even really young chicks stay on top of the hover instead of underneath. You can handle a lot of chicks under a relatively small hover but you do have to raise it as they grow.
