I'm trying to get a sense of how a Heroku web dyno compares to a standard vps.
For example, how many web dynos would be equivalent to a Linode 512? or a Linode 1024?
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You should start by asking Heroku themselves.
a Linode 512 is probably referencing 512mb of RAM, so you should go to the Heroku site and use their tool/s for how many dynos at 512 RAM and see the differences.
From what I can tell you concerning Heroku vs. a standard VPS, heroku is suppose to make the backend process more automated, whilst a VPS will be similar to having an AWS account.
In technical terms:
Heroku = PaaS standard VPS = IaaS