It is not the size of the squash, but the size of the plant that produces it. All squash get huge, and depending on conditions some turn into absolute monsters.

Uh oh. Hope this will work. I planted 2 in a 4-square place. So technically 1 per 2 squares. But they're diagonal from each other in a corner, and I'm putting the trellis on the corner... it's the north east corner of the bed so it gets sunlight all morning and all day.

Like this. X is the plant. 0 is open space.

Maybe I should eliminate one?

_____
:0x
:x0

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[0x
x0

If one is on an edge, you could always direct its growth out of the garden bed proper. I plant my bush squash planning on them spilling out/over the sides, and the vining types are given a generous trellis allotment.