Found this in historylegends video. Not sure where he got the data but he spends more time on this than me.
Marine Brigade: ~2000 troops
Territorial Defense Brigade: ~3500 troops
And adding the info from the Forbes article:
Tank Brigade: ~2000 troops
It is said that the Ukrainians organized 40000 troops into 12 divisions for the offensive (the 9 NATO and 3 home trained), one of them the 47th Mechanized Brigade. ~40000 / 12 = 3300-3500 soldiers.