Hello San,
LISA 7.5.2 has gone End Of Support mid-last year. From 8.x there has been a complete shift in licensing process as well which is honor-based rather than checkout count based.
First thing I would recommend you is to move to a later version of LISA that is now known as DevTest. Latest release is DevTest 10.0.
Coming to performance testing, there are two aspects to it: One is with respect to the size of payload and one is with respect to concurrent users against the target system.
In order to test with increasing size of payload, you would need to look at the hardware capacity of the server and distribution of LISA components over multiple machines. Network latency would also play an important part in that.
When you are looking to test the target system with increasing number of concurrent users then you would need to look at the licensing aspect along with the hardware capacity.
One simulator can handle maximum of 256 virtual users. Based on your requirement, you can calculate how many simulators would need to instantiate. For LISA 7.5.2, you would also have limitation based on your license as well with respect to maximum number if virtual users allowed in your license. You can get them increased by contacting CA Global Support team.