How to compare in the if block.

Hello.

I need to redirect a request depending on the request_length.

If the request_length is less than or equals 1000 bytes it's redirected to upstream X.
If the request_length is greater than 1000, then it's redirected to upstream Y.

Since the rewrite module doesn't support "less than", "greater than" comparison "If ($request_length > 1000) { ... }" is impossible.
Who can give me a hint how I could extricate myself?

Thanx.