vSRX is deployed using OVF template with all the predefined attributes, it will use 4GB RAM and 2vCPUs if you are using vSRX-S. I doubt that you can increase the memory, increasing in the ESXi shouldn't increase the actual memory.
Further on this topic does anyone know if you can get the routing engine to have more than 1g of RAM? I have a pair of vsrx servers (vsrx-l) edition running in vmware in a cluster but their routing engine only has 1g of RAM available. I am still testing things out but I have three ISPs for the office and each provides a full BGP feed for both v4 and v6. I only have 1 v4 session up now and the routing engine memory is almost full, if I could make the routing engine have 2-4g it would be good. The host has 128g of RAM and the vm has 32gb assigned to It, I have tried adding more to the vm but it doesn't seem to make any difference to the routing engine.