By using the idea that if something is inside you, you are then outside of it. Commonly used for drinking buddies when the drink is placed in front of you for enthusiastic consumption.
Alternatively chicks could use it to describe a guy they fancy getting a length from
*Returning from the bar*
"Get on the outside of that Fella!"
*watching hot bloke walking by*
"hey girlfriend, wouldn't you just like to get on the outside of that!"
By using the idea that a manhood has 'length', then this unit of measurement can be used to describe the act of sexual intercourse in terms of the number of 'lengths' used in the act.
To 'get a length from' someone would be receiving sexual action.
this can be expanded to getting yards based on the fact that most lengths are measured in inches.
"I've been getting a length from that hot bloke I met at the nightclub last week"
"did you hear that Susan has been getting a length from Tom?"