The resistance is proportional to the length/width ratio as you suspect. You can find resistance numbers for doped silicon, but the value depends on the doping

The resistance is proportional to the length/width ratio as you suspect. You can find resistance numbers for doped silicon, but the value depends on the doping level. So you can get a rough idea. Because resistances can vary 20% from chip to chip, analog circuits generally are designed so the ratio of two resistances controls things, rather than the absolute resistance. So you probably don't need to figure out the absolute resistance.