I have couple of mag stirrers here one is rated to 5 lt and will stir about 150 ml at room temp. the other is a hot plate and it will stir about 500ml @ 40c .
Look for the sh rating on the stirrer if has a rating of sh 2 is good for low volume sh 3 is better and sh 4 better again.
sh 4 stirrer will stir about 4 - 5 lt which will equate to around 200ml ish at room temp
I don't think a 1 lt will cut it for over 30 ml in a fairly wide beaker or flask