How to determine which size spoke wrench I should buy?
No this may not be correct.
The Wikipedia sizes (0, 1, 2, 3) are Park Tool specific sizes. Park Tool SW-1 is 3.30mm. Whether or not your multitool uses the same numbering as Park Tool is anyone's guess.
My understanding is that DT Swiss nipples all use 3.23mm (Park Tool SW-0, black tool). Of course your bicycle may have some other nipples than DT Swiss.
If building a wheel from scratch, I suggest choosing DT Swiss Pro Head nipples (they have a spherical seat as opposed to the cheaper but unfortunately more common conical seat), and remembering to grease the threads of the spokes well. Then the only spoke wrench size you need is 3.23mm.
To determine what spoke wrench size you need for an existing bicycle, invest in a caliper or a micrometer screw gauge.