The best choice is to get the cheapest SD card you can find (2 or 4 gig is adequate) and put the map on the card as
This way you have plenty of space for this and furure updates for your 255. It will really extend the life of your GPS in terms of future map updates as they become trivial.
The other thing is start removing other files to try and free up enough space (you can get these all back with the webupdater). You can get rid of
1. all the voice files except for the one or 2 you use
2. get rid of all the help files except for the language you may want to keep
3. get rid of any vehicles you don't use
4. get rid of any photos "jpg"s you don't use
5. get rid of the file gupdate.gcd (the update file that's left when you update formware).
You can download a smaller collection/version of the maps and use them. Full North America maps include US + CANADA + Mexico + a bunch of Islands and you could easily fit US or CAN (or perhaps both) in your device.
When the maps start to outgrow your available space, the SD card solution is better in the long run and small capacity cards a so cheap these days.