Why size things at all?

It's a home lab. Why not just give every VM "plenty" of everything and move on?

Because the constraint is real, and pretending it isn't is how you wake up to an out-of-memory kill at 3am. One host has a fixed amount of RAM, CPU, and disk. Every VM you start writes a cheque against it. The interesting engineering — the part worth showing — is deciding where the budget goes, and noticing which resource you'll run out of first.

Spoiler for this lab: it's always memory. CPU overcommits gracefully (cores are time-shared), disk is plentiful, but RAM is hard — when it's gone, something dies.

Why we use this: capacity planning is the least glamorous and most useful infrastructure skill. "Can we fit this?" is a question you should be able to answer with arithmetic before you provision, not discover afterwards. Every VM in this lab was sized by doing that arithmetic out loud.