I started with an average size dildo with lots of water based lube, and worked on length and depth of penetration first. Once I discovered how good that could feel, I increased the girth. You have to gain confidence that you CAN take it (without doing damage).
I found that a numbing agent like benzocaine applied just around and a little inside your anus can help you step up the girth little by little. Porn star bottoms are often "desensitized" to the point that they can't feel anything. Of course, for you and me, the point IS feeling a fat cock deep inside us!
A gentle, patient lover is probably the best way to learn the pleasures of bottom sex