I've got a pretty immense dildo collection (with lots of texture variation, most are on the firm side though); and I have tested this pretty extensively.
What I've come to is that I'm glad I have a variety to choose from. During my period I like thicker dildos (the big end of the Eleven
is my standard during that week).
If I want something phallic, I usually choose something about the girth of VixSkin's Mustang, but I prefer texture usually so Tantus's Tsunami
is the one that I'll reach for more often than not. But I also still love the perfect texture and firmness of my Gold Laced G
For non-phallic, size is less of a concern than shape. I love the ball end of my NobEssence Smooth S Shape (Seduction), which is about the same as the Pure Wand's big end.
I can detail my "scientific method" further if you're curious? But even after a lot of testing, the answer is still "it depends" and "I wouldn't give up any of them."