switch a home from electric heating to forced air

there is no way to answer that without a home survey. it would be best to call and get some estimates from local companies.

How big is a box? How big is a cloud.

One story? Two or more stories? Room for ducts? Unless someone can see the house and construction, it is impossible to say.

Lots of variables there. Get a few quotes... Its free to do so and will you an idea of costs.

While the furnace is the cheapest part of the deal, the duct work is going to cost some bucks.

I would also look into if your local utility companies offer some rebates on converting systems. It may not be much but every little bit helps.

Look into spending a high efficency unit too, a little more money now but at todays fuel prices will payback real soon.

Tom