I have the advantage of having spent the small fortune.....short that you do the following and be smarter than I was.
Fire form the brass, this can be by actually shooting a couple of correct cal. rounds you have.
Do NOT resize these cases, save for reference and use a belling die or even the rounded back of a nicely fitting needle-nose pliers to roll any residual crimp out of the case mouth. This only needs to be enough to be sure it is not rolled in even a tad.
Now trial fit whatever bullet dia.'s you have on hand,; most likely any jacketed sizes for caliber will 'fall right in'. Since they are pretty uniform and standard for caliber you now know quite a lot, but mostly that your cast bullets need to be 'bigger'. Many commercial cast bullets are bigger than jacketed but still may not be big enough., and to know what size you really bought you need to measure them, not just depend upon what the package says. I shoot quite a lot of 'as cast 'dia. bullets, not sized at all, because they fit the case mouth decently. Note here: if a bullet takes too much to fit them into the case mouth the upsizing of the brass will likely cause hard chambering and/or difficult extraction/ejection. Then you know that dia. is too big.
I like a snuggish 'finger fit' into the case mouth, or even lightly using the press to seat, as long as it will extract. As you can tell, this takes a little futzing about, but is really quite intuitive and simple.
But, you may ask, what if I have nothing to trial fit? Take anything cylindrical that you have about, and try the fit. This will give you a 'plug' to measure. If nothing fits, take a common hardware store dowel, cut a short length and chuck it in your electric drill or drill press and file it's dia. until you get that fit. Once you have it you are ever so much closer to picking a proper size bullet from the offerings. It is nice to try some actual bullets, and some companies will send you some samples, say .357, 358, and 359. Other than that, you will need to ante up for a mould and pick a sizer. I would almost say pick one a thou' smaller than you think you need because it isnt hard to lapp one out a tad to perfect and/or there are some manufacturing tolerances meaning the one you get may be a tad bigger than you need (say a .359 that ends up being .3595 or .360; it has happened).
FWIW, I happen to really like the Lee Push Through Sizing Die set up. It comes with some 'tumble lube' too. The fancy Lyman and RCBS are too rich for me anymore unless used on a G.S. table. Lee will also do custom sizes for a small additional fee.
