This is very general (if you can't get info on what is supposed to be on car)
The rack will typically be 1/2 to 3/4" longer (between tie rods) then the distance between the lower control arm mounting holes. Example if the lower holes are 18" apart, then a 18.25 to 18.75" length rack is generally used.
Rack ratio: sort of a personal preference, but typically the shorter the steering arms are you probably want a slower rack and vice versa. Ratio's have gotten a lot faster over the years and 4.0ish or faster is more the normal, but if you have long steering arms (around 5" verses 4.5") you might go up a little to a 4.4 to get similar speed as the short arm spindles.
For sure try to get a rack with the vertical slots, Just be aware there are different ends, some are more around the top of the shaft, some more at middle and some that are more below the shaft. Which one you need can vary depending on car. Now granted a lot of older cars don't need the slots because the tie rods are meant to be above the shaft so the old non clevis racks work fine for the most part.