Bullets are standard diameter, 0.224" but typically you use lighter weights. 35gr to 45gr. Most modern rifles chambered for the Hornet will stabilize a 55gr because they use a 1:14" twist barrel, but the small case capacity will mean velocity is very low. Barrels with a 1:16" twist will typically only stabilize up to a 45 or 50gr bullet.
Also consider a K-Hornet. It is the Hornet with a sharper shoulder, extending case life. You can get a gunsmith to ream out a Hornet chambered gun to the K-Hornet. A K-Hornet can still fire regular Hornet ammo, with little to no lose in accuracy, and it will give you the the fire formed cases you need for reloading.
Reloading dies? I use Lee, but have polished the inside sizing die with Flitz. (you can also use JB, but it takes longer). Flitz on a bore mop in a cordless drill for about a minute or so. Hornady, RCBS, Redding, etc, make K-Hornet dies.