If you look into the firmware source code you'll find the SPI data transfer buffer size to be 100 bytes (ard_spi.c:75). Because this buffer has to hold not only the payload data but also some additional information (about 9 bytes) you'll end with about max. 91 bytes of payload.
#define _BUFFERSIZE 100
Interestingly there is no check for that limit in the library on the Arduino side and in the protocol the message length is encoded in two bytes which would lead to the expectation that bigger buffer sizes are intended.