LOF( [#]fnbr )
For a file on the Flash Filesystem or SD Card opened as fnbr this will return
the current length of the file in bytes.
For a serial communications port opened as fnbr this function will return the
space (in characters) remaining in the transmit buffer. Note that when the
buffer is full MMBasic will pause when adding a new character and wait for
some space to become available so this function can be used to avoid this.
The # is optional.