All 3xxxx and 4xxxx registers use two byte, 16 bit data words, according to the Modbus standard.
However, whether any given device uses one word/one register (integer), two words/two registers (32 bit floating point, or long integer) or four words/four registers (64 bit floating point) depends on implementation.
Modbus doesn't define what kind of data resides in a register or how many registers it takes to get a complete data word, just the size of the register for a given address range.