More can be found by searching here, but the specific answers to your questions are:

Hood size needs to overlap the rising and expanding plume effluent from the hot pan surfaces. The quick and dirty (and reasonable as a trade-off) rule is that the hood should extend 3-inches past the range width on each side. This gets you a 56-inch hood that should capture most of the expanding plumes, assuming that the front to back dimension is adequate for the mounting configuration.

Flow rate (CFM) is dependent on the hood size, not vice versa. And it is better determined by the area of the hood entrance collecting aperture. Aim for an actual flow rate of 90 CFM/square foot of hood aperture. The blower rating that can achieve this is determined not only by 90 times the area, but by the pressure losses involved in operating the blower in a duct, with hood baffle losses, make-up air pressure losses, etc., and the candidate blower's fan curve. For another reasonable trade off, plan for a blower rated at about 135 CFM/sq.ft. of hood aperture.

And this requires that suitable low pressure loss make-up air is supplied. This is another topic to look into.

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