floor shifter hole size will be roughly 4.25 long and 3.75" wide.
I never have cut that big of hole on a 1948-54 PU. I use a 2 1/2" hole saw. Only the T5 rubber boot sticks through the hole, the shift cover is well below the floor.
I debated making a smaller hole. However, I need to be able to add tranny fluid through the shifter box instead of the fill hole on the side of the case. The fill hole on the side of a Camaro T5 case (what I have) is drilled lower than the S10 fill hole. It is made that way by design since the Camaro application has the case rotated about 17 degrees towards the driver.
In my truck application, the T5 is no longer rotated. Therefore, filling the case until it drips out the fill hole will result in UNDERFILLING. So, in order to add 5.9 pints through the shifter hole, I need to be able to remove the shifter base plate.
Now that I think about it a little more, I guess I could just park my truck sideways on a 17 degree hill and check the fluid level using the fill hole. Oh Boy!

That's just GREAT!

Where am I gonna find a 17 degree hill?
For an S10 case, the hole could be much smaller. Simply add fluid and check levels using the case bolts like normal.