You cannot size the AIP-SSM module based on users. The CSC Module (Anti-X) is sized based on the number of users.

For the IPS module, you need to estimate the throughput required. If its around 600/650 mbps at most, then the AIP-SSM-40 is fine. Else you need to propose a dedicated IPS appliance like 4260 or 4270.

http://www.cisco.com/en/US/products/ps6120/prod_models_comparison.html

Regards

Farrukh