I browsed through this thread last year before I bought my Ecoglow 20. I put 20 day-old chicks (10 red hybrid layers and 10 white leghorns) under it (maximum recommended) and was a bit worried that it wouldn't be big enough once they started growing. However my worries were needless. I had it in my sitting room though - so the background heat was not bad. All 20 survived and then once they were a month old I put them in a broody hen coop (ran an extension out so they still had the Ecoglow). It was really cold late March last year when they went out so I slung a couple of old duvets over the coop for extra insulation. They were absolutely fine and grew into happy healthy pullets.
This year I've got 50 day old chicks of different breeds arriving next week, so I've just ordered the Ecoglow 50. I'm figuring that they'll be pretty happy with a decent size pen - the ecoglow 50 and the ecoglow 20 in it. I'll keep them in my sitting room until they totally stink the place out, and then move them out to a hen hut. Fingers crossed!!!