I bought a whole bunch of 1/4"-20 25/64" 0.027" to 0.165" but when I drill hole the size of 3/8th, its too small for the rivnut to go. What drill size did you use?
1/4"-20 25/64" 0.027" to 0.165" is what you want
1/4"-20 is the most standard thread. 1/4-28 is a fine thread and not as commonly available for buying bolts from the hardware store (20 thread per inch vs 28 threads per inch)
The .027"-.165" grip range will be what you want for almost all the interior sheet metal on the Sprinter.