I agree, if you have the space to build, the more room the better. We used a pretty big dog crate, but by the time we had eleven chicks in there at 6+8 or 8+10 weeks...

It was getting a little ridiculous. We kept a close eye on the temperature the whole time, until it was time to shut the heat off. (They were brooded indoors, in my dining room. Lol)
Remember: you will want to start bringing the high temp in the brooder down 5* or so each week, eventually getting them acclimated to room / ambient temperature. Depending on the temperature in your garage vs. coop, you may need to make some judgement calls by the end of their brooding time. As in, hypothetically, it could still be quite cold at night in March, so you might keep them in the brooder longer, without supplemental heat, just at ambient garage temps so they can acclimate before sleeping outside in their new feathers. Or whatever the specific case may be.
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