We had one which was over 2 metres tall, but height depends on so many things, soil, fertility, pruning, species etc.

welcome to GoY
I also had a wild species broom that in the wild was no taller than 4 ft. The seeds I took produced plants of the same colour but it grew over 6ft. So as Owdboggy says it depends on lots of things.
There may be cultivated ones that are smaller. Check the label but be aware of labels that say growth in 10 years as the plant will then do the same the following ten yrs. Though broom desn'tt tend to be that long lived.