O.C. Choppers -- We're Making a Huge Bike ... FOR SHAQ!

The guys from O.C. Choppers are in the process of making one of their biggest bikes ever for Shaq ... and TMZ has learned it was customized for the big man with his NBA injuries in mind.

Shaq -- a huge bike fan -- asked for his ride to be taller and longer than normal to accommodate his arthritic hips ... and, of course, the shop obliged. We're told the completed bike will measure roughly 14 feet long (most are 10 ft.).

We're told ... since it's such a big project (due to the size of the bike) ... the guys are putting in a ton of extra work -- and it's all being caputured on tape for their new CMT show "Orange County Choppers."