input to IIR filter in Verilog

The basic purpose of dividing the total length in an integer and a fraction part is to define a scaling. Without a scaling, filter coefficients have no meaning. I'm not sure about the MATLAB definition of word length, I rather expect, it means total length = integer + fraction. But I think, you have a manual with the tool.

Assuming the real input data represents a range of -1..+1, then the filter must allow some headroom internally, depending on the filter Q and the used structure. Two or three additional bits left of the input range are usually sufficient. The other point has been mentioned by kaz. The filter may require additional fractional bits on the right, particularly if a high Q or a high fs/fc ratio is involved. But I'm confident, that MATLAB has a understandable documentation covering these topics.

In my standard filter model, the input data except for the sign bit would go into the fractional part, having the integer bits for the said overhead. But it depends on how you are scaling the filter coefficients.