[exim] Message size condition in a router?

Hello all. Is it possible to use a condition statement to evaluate message size in the routerstart? I looked in the documentation but didn't find any variable for message size in the router, only transport. Something like condition = "${if !>{$message_size}{1000k}}"

I use a 3rd party email sending service for 1 domain on a multi-domain server and have that router & transport working. But I wanted to add another condition to the router that if a message size is over 1Mb, I want it to skip the email sending service and continue to the default server for delivery. Thanks for any ideas!