In local.txt:
Memory=8000
or
Memory=8192
depending on which definition of GB you mean.
(Nowadays 8 GB = 8000 MB and you have to write 8 GiB if you mean 8192MiB.)
If you want a specific worker to have 8GB you add it below all the other lines in local.txt:
[Worker #1]
Memory=8000
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