Apache's limit to POST request size
The limit in Apache is set via the LimitRequestBody directive and defaults to 0:
This directive specifies the number of bytes from 0 (meaning unlimited) to 2147483647 (2GB) that are allowed in a request body.
Take a look at the full description of Apache's LimitRequestBody directive.
Besides that Rails may impose a limit itself (cant't tell though b/c I've never programmed in Ruby).