I've got a few of the above and want to make them 'bush out'.
Can I pinch out the growing tip ? and if so how big should it be when I do it ?
I wouldn't, or you'll lose the main spike. Modern varieties branch well on their own as the plant grows.
I asked this question once on a gardening forum (forgotten which) but I didn't get many replies. Someone said they didn't, someone said you should. I've pinched some of mine and I'll see what happens, but the ones I pinched (about 6" tall) are bushier than the others, but it's too soon to tell.