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Smallest that would fit a battery and a Heltec v4.
Live in a fairly small town in the southeast US but the local Amateur Radio club has a Meshtastic radio on a tall building in town and I was immediately able to see ~20 nodes and had 5 people talking in LongFast.
Wish I could have an outdoor pole mounted base station, but living in an apartment makes it hard. It seems to work okay on my 3rd floor windowsill though!
Link to case model: https://www.printables.com/model/1519914-heltec-lora-32-v4v3-pocket-pager-case/files
Yeah the device connects to your phone. But you can also get devices you can type on directly.
A base station? No, you just need one radio. It's a mesh network. Now it's pretty common for people involved in this to get two radios and mount one on their roof/on a tower/up a hill nearby - then they'll have a second device they keep near them. This gives you excellent coverage at all times, especially when you're indoors. These are low powered radios, even if the signal can go many miles in ideal conditions. But in a healthy mesh someone nearby enough could have a high up node, negating the need for you to have one.