Brussels are a long season crop. Check the listed days to maturity for the variety you want to grow. 90-150 days is typical with 90 being perfect conditions and an early variety. Never count on it. Always figure they will take longer as they often do.

Yes, they will fill a complete square. Many varieties in a good year will be very crowded when planted one to a square.

Typically I plant Brussels in the summer and harvest in the fall/early winter (I still have some in the garden and I am in Wisconsin). You can just direct seed them, no value in transplanting them since the summer soil is plenty warm.

The reason to summer plant and fall/winter harvest is that brussels produce sugars as antifreeze to protect against the cold. They don't do this until the cold temps set in. This makes them much sweeter.

Also, if you are getting toward the end of summer and you aren't seeing much in the way of sprouts forming, chop off the top of the plant. This focuses on sprout formation and ends vegetative growth.