Yes it is possible, I write 512 bytes to my Libelium micro sd module at a time.
I wrote a library that would take what I had buffered in my FRAM chip and then write it to the sd card in 512 byte increments.
The limit to how much you can write to a file is in the file. The sample file is only setup for a small amount of reading and writing. To increase the file simply add spaces to the file being sure to not disturb the start character. On my computer it looks like an L. Th println command is what you want to use for printing a line. print simply appends to the end of the file.
Hope that helps