This depends of the version of ESL you are using:

Up to version 5.45 of ESL:

The “PRGSIZE” parameter was not implemented, so the initial “pool” of committed memory is the amount (in K, or 1024 bytes) specified by the memory increment, the “MEMINCR” parameter. in the file “EslConfig.ini”, or environment variable of the same name. When the “MEMINCR” is not specified, a default value of 64K is used.  The ESL developer can simply change the size of the initial pool allocated by adjusting the “MEMINCR” value.

Version 5.46 of ESL and later:

The “PRGSIZE” parameter is used to set the size of the initial “pool” of committed memory in kilobytes. When the “PRGSIZE” is not specified, a default value of 512K is used.  The ESL developer can simply change the size of the initial pool allocated by adjusting the “PRGSIZE” value.