When should I consider resizing my Linode?

The short answer is "when your Linode is too slow for your needs" or "when you run out of disk space". The longer answer is a bit more nuanced.

CPU

For CPU usage, we ask that sustained use be within 80% of a Linode's total capacity. For example, a 4GB Linode has 200% total available. If you were to find that your usage was nearing 180% for hours at a time, you'd want to consider resizing to either a larger plan or a Dedicated CPU Linode.

Disk space

When you run the command df -h, you should have plenty of available space. If you're close to capacity, you should consider resizing to a larger plan. You could also consider using Block Storage or Object Storage instead of resizing.

Network Transfer

If your server transmits significant traffic across its public IPv4 address, you may find it exceeds your network transfer quota. You have the option of paying $0.01 for every GB of network transfer above your quota or adding another Linode to take advantage of transfer pooling, but you can also resize to a larger plan if you find that it works out better.

I'd also like to note that the opposite holds true as well. If you aren't using a lot of CPU resources and have lots of space available, you can consider resizing to a smaller plan.