The largest imaginable is probably somewhere in the millions of cubic meters. Bringing in futuristic mass-beam tech (hey, it's feasible with superconductors and you're under cryogenic temperatures already) and you can go bigger than any building built to date, perhaps even hitting the kilometric cube -- a billion cubic meters.

So we might say 30 meters is a relatively small one.

But even still, you are absolutely right that smaller ones are worth considering. In fact they are more worth considering because they can be done sooner. Every time you scale up by 1000, thermal transfer drops by 10. So if you just want to go from $22k to $2.2k, all other things equal, you can do this by going from 14 patients to 14000 patients.

The next wave of cryonics could take the form of relatively small (but still huge) urban cryo-centers that replace graveyards. A place like the UK where they are running short on grave spaces might be a good starting point for that.

Another important idea to look at is piggybacking cryonics onto other forms of cryogenic storage, or perhaps renting out storage in our cryogenic warehouses for other purposes as a source of funding.