go to the vmare machine's directory
and fine the file named "Red Hat Linux.vmx"
use your editor open this file , and fine the line begins with "memsize"
now your can change your memory size.
.encoding = "GBK"
config.version = "8"
virtualHW.version = "4"
scsi0.present = "TRUE"
memsize = "512"
MemAllowAutoScaleDown = "FALSE"