What device is it? If it's an old nuvi with only limited memory [some only had 256MB] you can use an SD card to place the map in a Garmin folder, named gmapsupp.img [path = sd:\Garmin\gmapsupp.img] With recent firmware for 2xx/2xxW you can have an extra map named gmapsup1.img in Garmin folder.

If you really can only have the detail map on your internal memory you can select just some needed map tiles to fit using BaseCamp and MapInstall.