It completely depends on the industry.
I work in media and journalism so my supporting statements tend to be shorter as most people will glance over the CV but look at the statement to see how you demonstrate the skills for the job.
A good rule of thumb that I've always is to only have a supporting statement that is one (as in just a front) page long.
For me, this usually translates into a couple paragraphs, not normally more than 500 ish words.
If it's for a teaching job, I'm not too sure how long or what to put in it. There are loads of websites that offer templates (just google how to write a supporting statement for .... jobs) for supporting statements that might be a good place to state.
It might also help to have someone read through your supporting statement who works in a similar role or industry to tell you what does and doesn't work, if possible.
Hopefully someone else gives you a bit more detailed info for you than I.