Currently (at least up to kernel version 1.1.68) input and output sizes have to be less than or equal than 4096 bytes unless the kernel has been compiled with SG_BIG_BUFF defined, if which case it is limited to SG_BIG_BUFF (e.g. 32768) bytes. These sizes include the generic header as well as the command block on input. SG_BIG_BUFF can be safely increased upto (131072 - 512). To take advantage of this, a new kernel has to be compiled and booted, of course.
The default timeout value is set to one minute (Timeout = 6000). It can be changed through an ioctl call (see section sec-ioctl ). The default number of retries is one.