Can I set the size of the cache memory?
There are no settings that deal with this. What is it you are trying to achieve?
The problem is that it takes a long time until Emclient starts after a restart.
It is a know issue that eM Client can be slow to start when installed on mechanical hard disks and especially so on 5400rpm laptop disks. There are currently no settings you can change that will improve that. The only option is replacing the hardware with an SSD.