I have a simple question that has been on my mind for a bit and I cant seem to find the answer. My question being: how do I know what size rod bearings to buy? I know that I have to use plastigage to see if the gap is within range, but I wonder, what if it is not in range? How do I know which size bearings to buy if the gap is too big?
Realistically, if your rod journals are worn, you'll need to take it to the machine shop to have the crankshaft inspected and reground if it can be done. The machinist (or shop) will then figure out if the crankshaft is serviceable. If it isn't, you'll need to get a new one (or a refurbished replacement). If it can be ground and made serviceable, your machinist will tell you what size of bearings to purchase. If you use a refurbished crank, the bearings you need will be undersized to accommodate. In SAE terms, you'd usually be looking at -0.010", -0.020", or -0.030" bearing sizes. When your machinist does the rods, they'll check the main bearing journals at the same time to ensure those are within spec.
Once you get your crankshaft back from the machinist and get the bearings of the size they state, then you use plastigage to see if everything is done right. If the machinist is worth their mettle (pun intended), everything will match up and your plastigage measurements will be spot on and within tolerance. You never really know though, and is why you always double check things when putting them back together. You never know if the machinist misunderstood you or you misunderstood them. Things happen. It is far better to figure out you have an issue before you put stuff together than to get the death knock on first startup.
As a side note, there are several things your machinist will tell you which plastigage won't. Things like whether the rod journal is out of round or is tapered. In most cases the machinist will be able to fix either of these issues, but unless you have a good set of micrometers, you might never know the difference. A good machinist is worth their weight in gold.