Plans

A plan is a preset size: vCPU, memory, disk, port speed and monthly bandwidth.

Add plans under VPS, Plans. Once you have one, Create VPS offers your plans instead of the standard sizes, and picking a plan also fills in its network limits. On a VPS's Manage tab, Resize lets you pick a plan too.

Changing a plan doesn't resize the VPSes already on it. Deleting a plan keeps their size.