Fire Forming Question

[Contenderizer]

I'm new to fire forming but am about to make the plunge. I will first be loading 22-Hornet ammo using new, unfired brass and firing same through a 22 K-Hornet barrel. The resulting brass will be formed to the K-Hornet barrel and I'll then be able to "kick it up a notch" the next time I reload same. My question is: Must I size the new, unfired brass, given that it will be rattling around in the 22 K-Hornet barrel anyway? I was hoping that I could check size by dropping a few un-primed bass in the barrel and, if they seemed to fit OK, check only for length.