The receiving partner should have a stretched urethra that is gradually trained to accept thicker toys.
The most common length is between 3 and 6 inches.
For some, this is part of the fun! It is best if you do not attempt any intercourse and penetration before your urethra is properly stretched with toys.
If you plan to do this, take your time and be careful not to stretch it to the point that it hurts or causes other discomfort.