Width is the most important measurement to get right, as you can always control how deep a dildo goes, but a too-wide dildo can be painful rather than pleasureable.
When a woman/vulva-owner becomes really aroused, their vagina naturally expands. The number of fingers that feel comfortably pleasurable at this super turned-on stage is a good guide for choosing the right dildo girth.
This is why we have classed our dildos in terms of fingers!
Some find that a dildo that’s completely straight can feel uncomfortable if it bumps against their cervix. If that sounds familiar, a toy with a rounded tip or gentle G-spot-friendly curve will usually provide a more comfortable, satisfying experience.