This is how you flash to eMMC easily

[dukla2000]

Sorry to reopen this thread so late, but...

I would like to try Mobian on SD card on my PinePhone, and then if I liked it, flash it to the emcc...

is there a specific size SD card I need to use to do it this way?

Once I flash it to the emcc, I'd like to be able to stick a big SD card in the phone, and mount it in my home directory... is that feasible?

The SDcard size is not too critical, anything from 8Gb upward will be OK  for testing. See other threads about SDcard performance, a card with good read/write speed of small files will give a much better experience when used fir the root file system. Samsung Evo+ works for me, spending more, or less, could be a bad choice!

And yes when you boot from eMMC you can mount an SDcard for data via standard linux functionality: I have a 128G Evo+ mounted as ~/SDcard.