How to read a plug base measurement
Separate the footprint of the external base from the size of the insertable section.
Find the measurement that describes the base
A round base may be described by one diameter. An elongated base can need both its length and width. If the listing gives overall width, find out which part sets that width before using it as a base dimension.
The Booty Bloom Small listing separates a 3-inch rose diameter from a 1.1-inch widest insertable diameter. The Simply Sweet set gives two dimensions for each base. Compare those figures as published rather than using the biggest number for every part.
Ask for a view that shows the widest point
Check a front and side view of the exact size. Ask for measured dimensions if photos lack a scale. For the separate question of width and usable length, see the compact plug measurements guide.