Engine bearing sizes can be confusing if you don't know what you're looking for.
This question is impossible to answer without information from you or your machinist. If you have a machinist, you should be asking them first. If they can't answer, well then you might need a new machinist! If your machinist is good at what they do, they will be able to understand what size you need and will either ask you to order the specific size for them to complete the assembly or tell you what you need if you're doing the assembly.
This is all assuming that your rods and main bore (where the main journals sit inside the engine) are within factory spec. If they aren't, then there will be more to consider than this answer here.
If you are starting off with a brand new OEM crankshaft, you can be fairly certain you'll need standard size bearings. It's always good to double check the journal diameters even on a new crank. But they are pretty dead on just about every time.
If you are getting a crankshaft machined (grinded, polished, etc.) then it will most likely get some of the material removed on the crankshaft journals. This causes you to consider an undersize bearing. Undersize is referring to the modified size of the crankshaft journal, NOT the bearing. The bearing is thicker when it's undersized, to close the gap between the newly machined crankshaft and the bearing.
First, you need to know what the journal specs are for your crankshaft. Most of our customers are using OEM crankshafts. Find the correct journal diameter for a completely stock crankshaft. If you sent your engine to a machine shop, they will most likely grind down the journals in increments of .001", .010", and .020". It all depends on how much the crank is out of spec. Usually, if you had to do any more than that, it might be best to get a new crankshaft. Under normal circumstances though, they'll usually take a crankshaft being machined for the first time down by .001". This means you need a -.001" undersize bearing. If they grind the crank down by .010", then you need a -.010" undersize bearing.
If you're not machining your crank, or you just simply don't know what size the journals are after machining, you'll have to measure for yourself. Using the proper tools, measure the diameter of the journals on the crankshaft. Compare that to the OEM factory spec. The difference is the amount of undersize and should correspond to the bearings you choose.
A micrometer is best to measure but not everyone has one as they are a pretty expensive and seldom used if you're a casual enthusiast. Plasti-gauge is a common way to measure if you're on a budget. However, you need the bearing itself to measure with plasti-gauge. You'll want to use what came off the motor originally to index what the new bearing size should be.
Typically, you will want .001" of bearing clearance per 1 inch of journal diameter. So if you have a 2.045" journal, like many of the Ford 4-cylinder crankshafts we sell, then you'll want .002" of clearance. If your journals measure 2.044", then you need a -.001" undersize bearing. If it measures 2.035", then that would call for a -.010" undersize bearing. If you're using plasti-guage, if the standard size bearing you removed from the motor had a clearance of .003 on the newly machined crank, then you know you need a -.001" undersize bearing to get it to .002" clearance . If you want an extra .001" of clearance, there are options for an oversize clearance bearing that will give you the extra clearance. But that is usually reserved for certain circumstances not related to any street performance build.