Hi Emine,
First, the customer must use the required hardware to deploy CWMS or they won't be able to get any support for the solution.
For 250 user system, they have to fulfill the hardware requirements defined in the Planning guide:
http://www.cisco.com/c/en/us/td/docs/collaboration/CWMS/2_7/Planning_Guide/cwms_b_cwms-planning-system-requirements-2-7/cwms_b_cwms-planning-system-requirements-2-7_chapter_00.html#reference_249B138B71324D19B09141D3849EC058
Manipulation of VM resource reservation is not supported and we cannot guarantee the stability of the system.
In addition, on the Dashboard, size of the system will state like this:
Size:250 simultaneous users
We don't have size of 150 at all (only 50, 250, 800, and 2000 users).
If you are referencing size being "Maximum Participants per Meeting:150", that is configurable in CWMS Administration > Settings > Meetings page.
I hope this answers your question. If not, please add more details.
In the end, do keep in mind that tweaking the VM resource utilization is not supported. VMs must be intact after deployed from OVA as the OVA defines all the required resources for them.
Kind regards,
-Dejan