You can guess or take the factory spec's (0.312") or slug the bore. To slug the bore, I use a muzzle loading round ball of pure lead, roll it over a piece of counter top to make an oblong shape slightly bigger than the bore, then tap it down the barrel. I use CLP or any good gun oil in the bore to ease it's passage and a piece of dowel rod to do the work once the ball is started in the muzzle. Measure the slug as it emerges with a micrometer across the impressed lands and you're good to go.
For hand loading, any good .32 bullet works well; in my experience, Hornady's XTP's are good, as are the Sierra and Speer offerings. I'v'e worked with 85 to 100 gr jacketed bullet with my 1925 vintage Winchester M-92, and also through a S&W hand ejector 4th model of the same vintage. The Model 92 will stay in an inch and a half at 50 yds with a peep sight off a rest. The Smith will do the same at 25 yds.
2400, Unique, Blue Dot, and Winchester 231 do well with either cast or jacketed bullets. For cast, I size them .312" and have good luck with minimal leading with either gun (and that M-92 has less than a pristine bore).
That Winchester 43 has a good rep for accuracy...Ken Waters did a work up on a Savage 23C (very similar to the Winchester), in May of '89 that really peaked my interest, and that you might find useful...his work was published by Wolfe Publishing in Supplement #4 to Ken's "Pet Loads" book. He too sized his cast bullets to 0.312" and reported excellent accuracy with Remington, Hornady and Sierra jacketed bullets. Good luck with your project.
HTH's Rod