Answer:
According to Garmin:
"As long as your nuvi or zumo has the latest version of software, it will be compatible with all SD and SDHC datacards. While SDHC datacards are available in a variety of storage sizes and speeds, we only recommend using a maximum of 16GB SDHC cards."
That FAQs refers you to the product documentation for specifics on if you model can accept SD or microSD cards.
According to the specifications page from the Owner's Manual for the nuvi 250s:
"Data Storage: Internal Memory and optional removable microSD or SD card. Data stored indefinitely."
So the nuvi 250s can accomodate either an SD card or a microSD card.