I generally get everything ready the night before or the morning of expected chicks. That includes the plate or pet heating pad. I have 3 plates (1 Brinsea and 2 Premier), and my pet heating pad collection is growing.
I would use the large. They grow FAST.
ETA: The large plate is more than twice the size of the small one. It would probably take 3 small ones to equal 1 large one.
You can use this with it, a lot of people do. http://www.amazon.com/dp/B0000BYEF6/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=3P7Y2QC3EAVJG&coliid=IWJX8Y0ATP0MJ&psc=1
I needed to set up some totes for small numbers of chicks indoors, and couldn't afford more plates at this time, so I came up with a solution that's working very well.
I got two small dog bed heating pads, under $20 each, and cut down cardboard boxes that fit them. I wrap the pads in paper...
I've been using one Eco-Glow and 2 Premier plates all spring now and they're still going. LOVE LOVE LOVE them!
I won't get any more Eco-Glows, due to the cheaper price on the Premiers. But I do plan to get some more Premier plates.
I don't know the lowest temp it can be used in, but I have one in an unheated barn. It got down in the 30's the other night and the chicks did great. The chicks were only 4 or 5 days old.