You want to know if 2048-bit certificate is supported in IMSS.
Yes, IMSS supports RSA 2048-bit. In fact, IMSS provides a sample certificate file that is tsmtpd.pem in <IMSS_HOME>/config. This file uses RSA 2048-bit.
The following third-party modules also support RSA 2048-bit:
- Postfix 2.3.8
- Tomcat 5.5.x
- Apache 2.0.x and 2.2.x