Well, there are a few I can think of....
A 4 x 0000 Graves.... Big blades... you can roll a racquet ball down the center with room to spare.
But my favorites aren't the speculum, they are the retractors that really gain respect....
The Leep lateral wall retractor can be handy, and a Sims vaginal retractor can help you see those nooks and crannies, but my favorite gravity assisted hands free retractor is the 3 lb Auvard Weighted Vaginal Speculum: http://surgical-instruments-supplies.medical-supplies-equipment-company.com/product/3lb-auvard-weighted-vaginal-speculum_27997.html , that is perfect for keeping a clear view while she lays in the stirrups on her back, or on her stomach, you get the contact picture, right... a vibrator applied to the outside in any position, but especially this one where it's in contact with the caudad portions of the labia majora, labia minora, and firmly applied to the periclitorral region, is virtually devastating. Also, there are lots of play games with this one, as turning it around has a certain sadistic pleasure for me, especially when I show it to her, and then proceed to lubricate it in front of her eyes. That large weighted bulbous end slowly forced in and then sliding in and out can be a great harbinger of orgasms. One game to play is practicing forced Kegel exercises with the bulbous end in while she stands... with a penalty if she drops it. One can tie a string to her nipples and the retractor with the obvious gravity induced consequences if she drops it....
And finally, the OSullivan OConner Blade http://www.thomasmedical.com/osullivanoconnerblade.aspx which lets you open and stretch to your heart's content and one size fits all.... hmmmmm maybe that's you can make all fit one size...
I'm just saying here..... but I have a devious mind. π