What's the ideal size for a Brooder box ?

I think Ive now got the temp set on the incubator and running correctly at 37.5C
so, now to the question of the ideal size for a brooder box ?

I had planned to make a box 4ft long by 2ft wide with 18" high sides out of 18mm ply which can be cut out of an 8' x 4' sheet with very little waste.

however, in the shed I have a wooden toybox, unpainted or stained,  29" long x 16.5" with 21" high walls   and am considering this as a cheaper option for my first attempt at raising 12 chicks

So,     what do you think ?

Hmmmmm

Well the plan was to make a lid (wood frame & wire mesh) and use either a heat lamp or one of those electric brooders (the sort of inverted hot plate thingy) (Warm NOT Hot)

Its going to be for a maximum of a dozen table birds at a time, I gather theyre ready to despatch at around 8 weeks onwards so..........Im open to suggestions

I have a spare coop mabye put a heatlamp in the coop and run attached so they can venture outside or stay in the warmth of the lamp

Suggestions welcome