Guys, at what stage in the loading process are you supposed to check the case length of your straight walled cases like the 45-70 and 450 Marlin? After firing, after sizing or after you bell the case prior to reloading?

I check after sizing.

Sizing can make a difference in case length, but the effect of belling is insignificant, and whatever effect there might be is reversed in seating and crimping, anyway. So, as Plainsman says, the meaningful point is after sizing.

Besides, although your trimmer pilot can be used reasonably effectively either before or after sizing, it is sized to be a tight slip fit for sized cases.