Thanks for the confirmation about the photos, Mary.
From the general "crispness" of the look of the millefiori PY weight, it could be from any of the (late) 50s, 60s or 70s periods! A photo of the base, at a slight angle, might help but it would really need a uv test with both longwave and shortwave bulbs to be sure.
If you have a longwave bulb ("blacklight") and it gives a definite green fluorescence in the dome then the weight is very probably pre-Caithness years. If the fluorescence is a "watery straw" colour or perhaps look like a "reddish green" then it is likely to be Caithness period. If there is no obvious fluoresence colour or it appears to be a "dusty pink" or a "dark straw" colour, then the Harland years are most likely.
But sometimes it is hard to say whether a non-green fluorescence colour ties in with the "rule of thumb" and therefore longwave uv alone cannot separate Caithness items from Harland pieces. That's where shortwave uv comes in - as, to the best of my knowledge so far, all of Paul Ysart's Caithness period weights, show as Blue under shortwave uv and no other period does that.
For size of paperweights, most of us are interested in the diameter at the widest point and the height.
Even within a given maker, "standard" sizes can vary. Many of Paul Ysart's weights were a "standard" 3 1/4 inch diameter but often "only" 3 inch. Others regular sizes were 2 1/2 inch, 2 5/8 inch, 2 3/4 inch or 2 7/8 inch. In the 30s, and maybe 40s, he made several magnum sized weights ranging from 3 1/2 inch to over 5 inch diameter.