Limit Words or Characters Per Post

Hi all, sorry if this has been asked 100 times before.

I did manage to get this sorted before, maybe a year ago or so, but I can’t remember how. I basically want to limit the words or characters for each post. I’m using a slightly modified version of the twenty twelve theme. The code looks a good bit different from previous default themes which is why I;m having difficulty getting this sorted.

Thanks.
Al.