The only way to be certain of your pellet size is to put them through a sizer.

The "competition grade" pellets may be more consistent than the cheaper offerings, but to be sure and confident, it would be best to mike a good number.

When I checked a selection of pellets a couple of years back, none of them corresponded in any dimension with what was printed on the tin, even the AA Fields. I measured head and skirt, both over two diameters, 90 degrees apart.

Then I bought a sizer from UK Neil!